For Auld Lang Syne

 is a quest in Fallout: New Vegas.

Pre-Quest Notes
Before this quest can be started an appropriate point in the New Vegas storyline must be reached so that you can tell the Remnants who to fight for, you must have progressed in any one of these quests: (You have to complete the part about the Omertas and be told to go after the Brotherhood of Steel, then Arcade Gannon will initiate the mission) Also, you must reach a certain level of 'trust' with Arcade Gannon before he will open his past up to you, the following are ways to gain trust:
 * For the Republic, Part 2
 * Render Unto Caesar
 * The House Always Wins
 * Wild Card: Side Bets
 * Become accepted with the Followers of the Apocalypse.
 * Take Arcade Gannon to the Crashed Vertibird, he will talk about how he hasn't seen one in a long time before quickly adding it must have been in a book.
 * Take Arcade Gannon into the REPCONN Headquarters where he will talk about the history of REPCONN and its research into plasma weapons after being bought by RobCo, with an intelligence of 6 you can inquire where he learned about this but he will dodge the question.
 * Taking Arcade Gannon into the Silver Rush in Freeside and he will start a dialog about plasma weapons. You can ask him how he knows so much about plasma weapons but he will dodge the question.
 * During The White Wash agree to blame the Scorpions (gang) for the water shortage.
 * Going to Camp McCarran after completing the quest There Stands the Grass and talking to Doctor Hildern about OSI will gain trust.
 * Going to HELIOS One and speaking to the Fantastic, then assuring Arcade Gannon you'll reroute the power supply to Freeside.
 * Take Arcade Gannon to the nuclear test shack at the Old Nuclear Test Site. He comments on how the Pre-War government "figured there was so much desert in Nevada they might as well turn it into a big dumping ground.
 * Visit The Fort while Arcade is your companion. He will start a conversation with you. Respond with "Let's just listen to him, figure out what he's up to, and get out".
 * Having ED-E as a companion before talking to Arcade instantly gives you the quest should you choose all the first options towards the end of the game.
 * Betting on Radscorpions in The Thorn with Arcade Gannon following you, make him comment after the fight with latin aphorism. Non-functional on PS3
 * After completing The White Wash arrest Anderson but keep Westside supplied with water

Walkthrough
The player must visit various locations to recruit 5 old members of the Enclave to aid in the final battle at Hoover Dam. The player may visit the remnants in any order. The 5 remnants are:
 * Note: These five members are can be killed, be careful should you run into them beforehand, or this quest cannot be completed.
 * Note: If you don't want to be stuck without dialogue with Arcade Gannon at the end of the quest, after asking each of the remnants member to join, wait for Arcade Gannon to catch up, and he will be talking with you about the opinion of the current remnant you are talking to. If you skipped the talking with Arcade Gannon during this, there will be no option to choose for Arcade Gannon to stay or back to freeside at the end of the quest.
 * Doctor Henry at Jacobstown
 * Cannibal Johnson at Cannibal Johnson's Cave
 * Judah Kreger
 * Orion Moreno
 * Daisy Whitman

Guess Who I Saw Today will need to be completed before Henry will agree to go to the Remnant's Bunker. Once all 5 remnants are convinced, head to the bunker to instruct them as to which side to support in the battle. The passphrases each remnant gives the player upon recruiting them will be used to open the bunker's main door. The player needn't remember them or unscramble them, once you've sent all 5 there the full sentence is there as a conversation option when you interact with the entry terminal.

Upon entering the bunker, head to the briefing room in the back and talk to Judah. You will then have to make a choice: have the Remnants support the NCR or Caesar's Legion in battle.

NCR
Even if you are working with Mr. House or Yes Man, those quest lines still need them to side with the NCR. if you ask them to side with the NCR, Orion Moreno will leave because he bears a strong hatred towards them. If you leave the briefing room and talk to him in the hangar, you can convince him to stay and fight with a Speech score of 80. If you cannot do that, you will have to fight and kill him as he will immediately turn hostile. You can also tell him, with a high enough NCR infamy, that you do just as much bad as good for the NCR. Regardless of how you handled that situation, Judah will tell you to talk to Daisy for Power Armor training and can use the armor you can loot off of Orion (if you killed him) or Daisy's, as she says it makes the cockpit too crowded if she's wearing it.

Legion
Should you ask them to side with the Legion, Cannibal Johnson will leave and there is no way to convince him to stay. You can then talk to Orion Moreno for training in Power Armor and will get Johnson's suit since he didn't take it when storming out.

Judah will ask you to head on out and finish up your business while they plan their attack. When the battle at the dam takes place, they will be dropped in by the Vertibird and battle whichever side you had them fight against. When you leave the bunker, Arcade should approach you and ask what you think he should do. He wonders if he should aid the remnants in battle and follow in his father's footsteps, or stay out and make his own path. If you pass a speech check of 80 and remind him of his duty to Freeside each time he talks to you about the Remnants, then have him not join the remnants he will give you his father's Tesla Armor and helmet. No matter which option you choose, this will complete the quest and will cause you to permanently lose Arcade as a companion.

If you are working with Yes Man or Mr. House, then during the battle of the dam you may have to fight the Remnants if you recruit them as there is no option for them to support you. Although, if you pass a high speech check to the guards in the control room convincing them to leave, then remnants (and all NCR troopers as well) won't turn hostile. Cannibal Johnson and/or Dr. Henry will follow and assist you on the attack to the Legate's Camp. Johnson will sometimes follow you into the camp as if he were a companion and aid you in killing Lanius.

Bugs

 * If the player is given a set of Remnant Power Armor and Helmet from Daisy Whitman and already has another set in their inventory, the game cannot recognize the sets as being different. This causes them to appear as one set in the player's inventory, but you can drop or store them twice, because you actually have two sets.


 * The ripper is Arcade's default melee weapon. A similar bug can occur when entering casinos on the strip and having all weapons removed; sometimes a weapon your follower was using will be placed in your inventory. In this case your followers weapon can be dropped, but not picked up again. It will appear as, "Steal _____" with no item name given.


 * Player is unable to leave the briefing room (door is locked and requires a key) after using speech skill to convince Orion Moreno to stay. One work around is to pickpocket the lock down code from Orion Moreno. Another work around is to kill all the remnants. They will not show up at Hoover Dam but Arcade Gannon will not be hostile.


 * Player is unable to leave briefing room after killing Orion Moreno due to the key being removed from inventory upon quest completion. No other NPCs in the room have a key, and the only option is to reload a save. the Remnants will still help you during the final battle, but the quest will never be completed. Alternatively if you do not mind failing the quest and are stuck due to this glitch you can get out of the room by shooting at them as soon as you reload into the room. This will make them hostile towards you and let you keep the key. Choose this route if you did not save in a long time but don't want to loose hours of work.


 * After completing the quest, Gannon will ask whether he should stay with the Enclave Remnants or return to the Followers of the Apocalypse, but the player will be unable to answer. The player then will be unable to open Gannon's follower wheel(Note:this can be fixed by dismissing him as a follower then recruiting him back and before you do that, pickpocket him for the stuff you gave him since when you try trading with it, it won't show up in the trading screen.)
 * This is due to a counting 'error' done by the game which determines whether you would like Arcade to be more Pro-Remnant-like or Pro-Follower-like. This counter is started by the dialogues initiated by Arcade when calling each one of the former Remnants to the meeting. Every one of the Dialogue options in those conversations counts towards either the Followers or the Enclave. To get his dialogue after leaving the bunker however you will need to have at least 3 points for either one of them. However it is possible to miss one of the conversations by fast-traveling immediately because Arcade will not follow you right away, making it possible to skip his dialogue and talk to the next Remnant without talking to Arcade first. This can either be fixed through the G.E.C.K. tool by changing the number of points necessary to become able to answer him or through the console by typing: set "16117f".nProFollowersCount (if you want him to stay with the Followers) OR nProEnclaveCount to 3(at least!!). The second option however will deactivate all Trophies/Achievements.
 * Player is unable to start quest, despite having exchanged dialogue with Arcade that should allow the quest to begin. The quest does not appear in the pip boy quest list. [have you progressed far enough down the main story line to unlock it yet?](Console command: "setstage 00157e60 10" activates the quest.). If all other suggested options fail or are unavailable you can also talk to Colonel Moore during the For the Republic, Part 2 quest. This has caused Gannon to initiate the quest.


 * Gannon will leave your companionship, and there will be no dialogue to recruit him back. This is bug has been documented with Gannon outside of the quest, firing ED-E or Rex may restore the dialogue to hire him in some cases.
 * It is likely a bug and a quirk of Gannon's personality. Gannon makes no secret of his dislike for ED-E in conversation, it is likely he refuses to join you while ED-E is a companion purposely. The fact that the effect extends to Rex is likely a bug.


 * Gannon will not give the dialogue for activating the quest no matter how many of the locations have been visited, making the quest impossible to start. ist. [have you progressed far enough down the main story line to unlock it yet?] If this happens and you are playing on the PC you may open the console click on Gannon to select him, then type AddScriptPackage 166d95. This will cause him to immediately approach you and give the quest.Or use ( setstage 00157e60 10 ) this will start the quest immediately.


 * The dialogue will be short with Arcade Gannon after settling the task with Orion. There is no fix for this. The bug also makes it so that you cannot get Gannon's Tesla Armor upon existing the Bunker.


 * Upon entering the Remnant's Bunker, Daisy Whitman will randomly die, causing the mission to fail.


 * Doctor Henry will only agree to go to the Remnants reunion after completing his research on Nightkin, sometimes after handing in the chewed stealth boy and telling Lily to help, he will never use the Stealth Boy MK II on Lily. Thus making his research incomplete and making it impossible to get him to go to the Reunion. (Doctor Henry started his research after "Nothing But a Hound Dog" was completed) (ps3)


 * When the Enclave Remnant appear during the fight at the Hoover Dam, if the player is sided with Mr. House or Yes-Man, the Remnant will be hostile. [not a bug. you only have options to tell the Remnants to support the NCR or Caesar, not for yourself. obviously they are going to attack you]
 * The Remnants will not go hostile if you asked them to assist the NCR and if you got the control rooms guards to leave using a speech check, as they will not know about you installing the override module into the Hoover Dam computers.


 * If you are already wearing world looted remnant armor when you talk to Daisy to get her's then it will not spawn in your inventory.


 * When the Enclave remnants appear at Hoover Dam, Orion Moreno will be begin slowly walking towards the visitor's center rather than follow you, and a 2nd Doctor Henry will join you, wearing the Wasteland Doctor Outfit. The reason for this is unknown.


 * When Arcade Gannon joins the remnants he will get his Tesla Armor after you talk to the remnants he was near the ladder when spoke to he walked through the ladder and disappeared he was later found in the debriefing room but he was glitching into the room ( he was flickering in and out) unable to speak to him but if you open pip boy or vats him he returns you can talk to him but he always goes through the door and then glitches again.


 * When you leave the Remnant's Bunker, Arcade Gannon won't ask you about your opinion about his role in the fight, but simple just keep following you, still making him your companion, and making it impossible to obtain the Tesla armor.


 * When leaving the bunker, Gannon may enter a passive state in which he will not initiate the conversation about his role in the fight. He will also not engage in combat and activating him results in dialog instead of opening the companion wheel.  Choosing dialog options to give commands, with the exception of dismissing him, will trigger the appropriate verbal response but will not result in any action on his part.  This means that while he can be sent back to the Old Mormon Fort, his inventory becomes inaccessible and you cannot retrieve anything weapons or armor he was carrying.  Upon returning to the Old Mormon Fort  he will not offer the option to rejoin you.
 * A possible partial fix is to take Gannon and back to the Lucky 38 (or the Old Mormon Fort but I haven't confirmed that). Once you are there dismiss your non-humanoid companion (Rex or ED-E) and then dismiss Gannon.  You should then be able to recruit Gannon once again as a normal follower. Though he still may not initiate the dialog regarding his role in the battle, he should at least give you access to the companion wheel again.  (tested on 360)

Behind the scenes

 * The name of this quest is in Auld Scots. The translation of this quest is "For Old Times' Sake"
 * The name of the quest may relate to the 1788 poem by the same name, written by Robert Burns, the lyrics seemingly relate to the remnants situation. The lyrics can be read here