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.