You'll Know It When It Happens

You'll Know It When It Happens is a main quest in Fallout: New Vegas.

Detailed walkthrough
The Courier will be directed to speak with Ranger Grant in order to make preparations. Speaking to him will trigger a wait period, after which it will be time for the speech. He will allow the player character to make a security sweep before the speech begins. If the player character is not in good standing with the NCR he will not give the Courier the itinerary for the speech and the player character will not be allowed to draw their weapons even if intending to kill the sniper or engineer. The Courier may ask Grant for access to all areas of the dam in order to do a proper search. At this point, the player character may either complete the optional objective or skip to the speech.

Optional
Investigate the area for clues. There will be no waypoints to direct you, so you must locate them manually.
 * On the second floor of the visitor center you can talk to Allison Valentine, who is looking for her engineer friend. He has been missing for several hours.
 * The engineer's blood stain can be found in the supply closet in the Hoover Dam visitor center, on first floor behind the staircase. It requires a Perception of 6 to examine.
 * Hack the computer terminal (You will need 50 Science since it's Average security) in the visitor center to discover an unauthorized access ID (Sgt. Macovitch). The terminal is unowned, so the soldiers will not become hostile.

Note: If you have Boone as a companion, when you walk up the ramp on the visitor center exterior towards Ranger Grant he'll say something about if he were the sniper, where he would shoot from. He'll tell you 3 locations (he is correct about the last two):
 * the ridge to the left of the road;
 * the tower behind the stage;
 * the roof of the visitor center.

The speech
Once you speak to Ranger Grant again, the speech will begin. Kimball's Vertibird, Bear Force One, will land on the roof of the Visitor Center. Kimball and four Rangers will disembark, then move to the stage for the speech. In order to hear the entire speech, each assassination attempt must be dealt with quickly and covertly; alternatively there are opportunities to scuttle the speech early to get the President to safety.

Keep in mind that even if you asked for and received clearance to enter "all areas" from Grant, entering the backstage area or going onto the stage with the President will cause the NCR to become hostile.

There are three different assassination attempts:
 * A bomb is placed on the Vertibird by an engineer;
 * A sniper, dressed as an NCR Ranger, will appear atop the guard tower behind the stage set up for the speech;
 * A Legion soldier, disguised as an engineer, will run up to the President and try to assassinate him with a knife. (if you speak to Ranger Grant and tell him about any of the above two attempts, this can not be avoided).

If you received this quest from Yes Man or Mr. House, the staircase leading to the roof of the visitor center will be locked. However, so long as you're in good standing with the NCR and asked Ranger Grant for access to all areas, you can go up using the ladder on the outside of the building. Private Jensen will be guarding it, so an NCR disguise might be necessary to bypass him. A disguise can be found in the storage closet underneath the stairs in the visitor center. You can also talk your way past him with a Speech skill of 50. The same can happen if you have a mixed relationship with the NCR even if you attain this quest through the NCR.

Once on the roof, depending on how quickly you get up there, you may see an engineer tamper with the Vertibird. "Activate" the Vertibird to examine it, which will reveal that a bomb has been planted on it. An Explosives or Repair skill of 50 is required to remove it. Failing that, you'll need to get the detonator from the engineer. This can be done before or after the arrival of the sniper. He'll head into the crowd watching the speech, so you'll need to find him. He is on the left side facing the podium. The right (or left, depending on whether you are watching from the visitor center or the guard tower) engineer is the only one who will talk to you, and is the only one who won't clap. He will also move around occasionally, as noted in some cases. If you have Rex in your company he will detect the Legion engineer in the group, growling at him to point him out. Talking to the engineer will turn him hostile, thus canceling the speech. If you have the Animal Friend perk, you can take the search dog from the Ranger in front of the visitor center. The dog will also sniff out the disguised engineer.

Whether or not you have been able to defuse the bomb, you still need to stop the assassin at the guard tower. You will see him throw the real guard off the guard tower and take his place, unless you're already there, in which case he'll attack both you and the real guard on the spot. If you've missed the event, there are several ways to tell if the assassin has come. First and foremost, the ranger's body will be on the ground in front of the tower. It can be examined to determine that the ranger was killed recently, but cannot be looted until after the quest is completed. Second, the assassin, though dressed similarly, has a visible face. The ranger wears a face wrap. Finally, the ranger will remain in position near the radio on the tower, while the assassin will pace back and forth. About halfway through the speech, the assassin will line up his shot, firing after a few seconds.

Finally, the Legion engineer may attempt to stab Kimball personally. This won't happen if you covertly disable the bomb, since the engineer will still try to detonate it. Assuming he does attack, kill him and the final assassination attempt will be foiled. Kimball will return to his Vertibird and depart, completing the mission.

Alternate completion methods
There are a few other methods with which you can end this quest. However, each of these methods yields a different ending to the quest. Note that, no matter which way the quest is solved (as long as NCR doesn't turn hostile on you), you will receive the Eureka! quest (if you are siding with the NCR). All of them involve canceling or interrupting Kimball's speech.
 * You can speak to the engineer before the Vertibird even lands; he will respond aggressively, and you can lead him to attack by expressing your suspicions and offering to call the MPs or take him away yourself. Kill him and the speech is canceled, the Vertibird leaves, and the quest is completed, even though Grant won't talk to you. The Vertibird at least needs to be visible for this to work, as the engineer will simply respond as any other engineer would prior to its appearance.
 * You can report the existence of either the bomb or the assassin to Grant, provided you've dealt with one or both already. You don't need to disarm the bomb to report it in, merely find it, but you will need to kill the assassin to report him in. Once reported, Grant will cancel the speech and usher Kimball to safety by various means.
 * If you deal with the sniper and report the bomb in, it will trigger a scripted event where the engineer attempts to stab Kimball, only to be dispatched by the Rangers.
 * Disabling the bomb and reporting it without getting rid of the assassin will cause him to take a shot at Kimball, only to miss and hit a Ranger instead. Kimball will flee to safety. This is an easy way to obtain the NCR Ranger combat armor, provided you're sneaky enough to loot it without being seen. The NCR soldiers will turn hostile if they catch you. You can also use an NCR disguise to get on stage without suspicion.
 * Reporting the assassin without disabling the bomb will cause Grant to sweep the area again, causing him to find the bomb and defuse it. The engineer will also try to kill Kimball, so be ready to kill him first, as nobody in the crowd will stop him.
 * If you talk to the assassin on the tower after he pushes the ranger off, you can question him about it. He will then suggest going down there, but if you tell him not to, he will turn hostile and you can kill him. After that, if you report to Ranger Grant via the radio on the tower, tell him what happened and he will evacuate the president. As long as you haven't completed the optional objective with the dead engineer, the engineer won't stab Kimball and the bomb won't go off. Kimball will then fly off and the quest will mark complete. This can be done even if you haven't defused the bomb.
 * You can pickpocket the engineer and steal the redundant failsafe detonator that he has in his pocket. After that you can show your finding to Grant. He will send two rangers to arrest the engineer. Before the rangers come to arrest him, it is scripted that the engineer will take a knife and kill one of the rangers before another shoots him dead. The speech will be canceled and the quest will be successfully completed.

Behind the scenes

 * After killing the Legion sniper on the guard post and using the radio to report to Grant, one of the dialog options ("Everything's fine up here, thank you. How are you?") is a reference to dialog said by Han Solo (played by Harrison Ford) from Star Wars Episode IV: A New Hope when he and Luke Skywalker are saving Princess Leia from the Death Star and are questioned by Imperials over the intercom.
 * A majority of this quest was implemented by Charlie Staples.
 * The name is a reference to a song by the popular singer Frank Sinatra, "You'll Know When It Happens" (without the first "it"), like many other quests in New Vegas.
 * The president's Vertibird being named "Bear Force One" is a pun on the President of the US's aircraft "Air Force One" and the NCR's coat of arms animal being a (two-headed) bear.

Bugs

 * After Colonel Moore instructs you to speak with Ranger Grant in the Visitor's Center and starting this quest. If you speak with him before he initiates dialog the quest will be broken and cannot continue further.
 * The solution is to go to the Dam's Visitor's Center, walk to the desk, and wait for Grant to come talk to you.
 * You can also use the kill and resurrect commands on Ranger Grant, then use the command and Grant should then initiate dialog.
 * The more elegant solution to this shy Ranger Grant problem is to get close to him, open the console, target him via the mouse or via command then enter, then type disable and enter then enable and enter. It gets him to lose the shyness and doesn't cause auto fails of related quests via killing the character and the game catching wind of it. Don't forget to exit the entire game and re-load if on PC to re-enable achievements after using the console.
 * President Kimball and his escorts sometimes fail to board the Vertibird for departure, regardless of the method of quest completion. They will either stand next to the Vertibird, or mysteriously explode within the Visitor's Center.
 * Another solution is to speak with Grant to begin the Vertibird arrival, then go to the roof and assassinate the engineer without being seen (otherwise the NCR will become hostile). The Vertibird will land and immediately take off. You will not be able to report to Grant, but the quest will be flagged as complete.
 * Yet another solution is to ignore the Engineer initially. Instead, discover the Legion sniper and go to the tower to talk to him and expose him. Use the radio to report to Grant. You have plenty of time to run back to the stairs leading up to the Visitor's Center and intercept the engineer as he runs down to attack the President. Once the engineer is killed, just wait for the President and his security detail to go up to the Vertibird via the exterior ladder, and fly off. You do not need to have personally discovered the bomb on the Vertibird (which should have presumably been placed), recovered the detonator from the Engineer's body, nor make additional reports to Grant. This is possibly the intended solution if someone has a limited reputation with the NCR and cannot go up to the Vertibird landing pad without making the NCR hostile.
 * Sometimes, even though you start the quest by speaking with Grant, after you investigate the closet and check the terminal, going up onto the observation deck will not trigger the Vertibird landing and Ranger Grant will not initiate dialog any more.
 * Note: If you are unable to find Ranger Grant on the roof of the observation deck, sometimes he may be found waiting by the Visitor's Center staircase outside. If this happens you must revert to a previous save from before entering the building and speaking to Colonel Moore.
 * If you start Wild Card: Finishing Touch after starting this quest, you won't be able to talk to Ranger Grant (only one dialog option - Goodbye) and thus unable to start the event. Your only option is to finish Finishing Touch and let the president die.
 * On the PC, opening the console, selecting Ranger Grant and using the command followed by the  command is also a possible fix.
 * If you approach him from behind seems to fix this bug, as it makes him start the conversation.
 * Shooting either him or an NCR ranger near him will cause him to run back to the Vertibird. His security will open fire on you, but will stop if you move so far away. Kimball will leave in his Vertibird, and the legion dressed as an NCR engineer will be gunned down as he tries to attack Kimball. Within a few seconds after his departure, you will have completed the mission.
 * Though there are three assassination attempts, not all of them may be triggered during the quest. The bomb on the Vertibird will usually go off unless you deal with it, but the sniper and assassin in the crowd may not act.
 * If you disarm the bomb on the Vertibird and then immediately run to the tower where the sniper spawns, climbing the ladder to the top of the tower may initiate the following sequence. When you reach the top, you will hear the scream of someone falling, but the Ranger sniper is still there alone and fine. If you wait through the speech on the tower with the Ranger sniper, nothing happens until the very end of the speech where a shot will ring out, President Kimball will disappear, the Ranger sniper on the tower (next to you) will disappear, and you will fail the quest (the only dialogue option when talking to Colonel Moore afterward is to tell her that the President was killed by a sniper).
 * If President Kimball finishes his speech and successfully makes his way back up to his Vertibird, he may start walking endlessly against the ledge of the roof of the visitor center away from the Vertibird, and never leave. To fix this, talk to Ranger Grant regarding the bomb or the sniper, and this will initiate the escape sequence for President Kimball, forcing him to depart in his Vertibird.
 * After killing the sniper, report back to Ranger Grant. His first dialogue will be normal but his second sounds like it's coming through an intercom.
 * If you receive the quest from the NCR quest line, you can request Ranger Grant to have full access, and it should be granted. As soon as you're ready to start up the quest, go outside and speak with Grant and tell him you're finished preparing. Immediately go onto the roof of the building, there should be a couple of NCR troops awaiting the President, and one engineer leaning against the side of the doorway leading to the visitor area. If you have a good sneak skill, you can hide on the other side of the wall. After the Vertibird lands, and the closest NCR troop goes downstairs, the assassin in disguise will start walking toward the back of the Vertibird. If you're hidden and you use a silenced weapon, you can kill the assassin before he places the bomb. This will cause everyone in the crowd to panic, as well as the President when he exits. A short time later, he should take back off and it will say that you successfully finished the quest.
 * It is possible that before reporting to the Ranger for the first time, many members of the audience will be stripped bare of all of their clothing and will stand there in their underwear during the speech.
 * Sometimes the Engineer glitches and never runs towards the stage: the mission is still accomplished and you may see the Engineer still 'watching' the speech later on in the game.
 * The PS3's lockup bug where the machine freezes up after extended periods in the open wasteland is especially prevalent during this quest. Simply restart the machine and reload the autosave and re-attempt the quest to get around this.
 * When you try to pickpocket the engineer and take his detonator and keys, without disabling the bomb in the president's Vertibird, it still would explode. even after you protect the President from the sniper and the assassin. the trick of this glitch is to kill the engineer right after the Vertibird landed, causing the President to flee safely before he is able to start his speech.
 * When you start the quest talk to Ranger Grant outside tell him you are ready then run up to the tower behind the stage climb the ladder then climb down and it should say quest completed.
 * It is possible that Kimball's Vertibird may land and then suddenly take off. After this, the quest will be rendered complete without you having to stop any assassination attempts.
 * After initiating the dialog where Ranger Grant tells you to get some rest before the President arrives, the game may crash after fading out. (Possible fix) Wait to a certain time (i.e. 10:00 PM) then talk to Ranger Grant to proceed with the quest. (Waiting until 10PM confirmed fix on PC) (Possible fix) Wait multiple one-hour increments. If waiting is disabled, find a bed and sleep in one-hour increments.
 * When the Vertibird is flying to come over to the base to land. Press X and you'll complete the quest without having to protect him.
 * It is possible that when climbing the ladder to the tower with the sniper assassin while having ED-E as a companion, that all NCR troops will become hostile to ED-E. Easily fixed by telling ED-E to go back to the Lucky 38 as of patch 1.4.0.525, since doing so will despawn it on the spot. This also forced the rangers to evacuate the President and complete the mission.
 * Sometimes the Legion engineer won't attempt to stab Kimball.
 * It is possible to encounter Grant outside the Hoover Dam area when he gives this quest, such as Black Mountain, causing it to fail instantly after being taken (since the Courier is not allowed to leave the Dam area).
 * When the engineer rushes to stab Kimball, attempting to kill him before the Ranger dies will cause Kimball to glitch and continuously run in place. This makes him unable to retreat back to his Vertibird and the quest impossible to complete, forcing the player to reload the save.
 * Sometimes if you intercept the bomb then rush to the guard tower, President Kimball will not start his speech and render you unable to complete the quest.
 * Sometimes, if you kill the engineer, Ranger Grant will freeze with his face in a grit-teeth glare and will not be able to be interacted with anymore, nor will he move or do anything. Kimball will also run back to his Vertibird and take off successfully and safely regardless of whether or not you've disarmed the bomb.
 * After talking to Ranger Grant, the president will arrive and all the doors will say [key required], and you will be stuck in the Hoover Dam building.
 * To get to the roof, open console commands and type . This will send you next to President Kimball. If he does not start his speech, you can pickpocket the Engineer in the crowd. He is carrying a bomb. Report it to Ranger Grant and make sure Kimball gets to his aircraft safely. After he departs, you will have successfully finished the quest.
 * Talking to Ranger Grant will only have the option "I need to prepare first" making it impossible to finish the quest. This can be fix by open console commands and type on him.
 * If the player's reputation with the NCR is Soft-hearted Devil, once arriving at the dam, all armed NCR soldiers will attack you, killing any of these soldiers will instantly fail the mission.
 * If the president has fled but everyone is stuck thinking he's still in danger use console command
 * If you head into the Supply Closet after talking to Allison Valentine, you will sometimes experience a glitch were your game will freeze and if you reload the auto-save it will tell you to report back to Cassandra Moore saying that the President has been killed.
 * It is possible for the game to glitch after disarming the bomb and killing the sniper. The engineer will not attack, and Kimball will not move and Grant will not speak to you after the speech ends (no matter how long you wait). A work around is to take out the engineer (VATS may be necessary, as bullets will sometimes pass right through him). Kimball will then run toward the Vertibird, although several characters may glitch and block his path, forcing you to push them out of the way and allow him to pass. If Kimball makes it to the Vertibird, it will take off as normal, although Grant will still not speak to you.
 * Sometimes after initially starting the mission, there will be a Great Khan outside of the Hoover Dam main entrance. The NCR troopers will start firing on the Great Khan and kill him or her. This will cause the mission to immediately fail.
 * In some rare occasions, when the engineer becomes hostile, everyone becomes stuck in a panicked state without moving, thus President Kimball and the guards do not go to the Vertibird. To fix this, you will need to load a save from before the quest began.
 * If you jump at just the right position when the Presidential Vertibird leaves, you can get stuck inside the Vertibird, and fly off with it. Eventually, you will either clip out of the Vertibird or the Vertibird will disapear. In both events you will die from fall damage.