How do I perform the fourth-day glitch in both versions of The Legend of Zelda: Majora's Mask? I'm very interested in it, and I've seen several videos on it, but I just can't seem to get it to work. The more detail, the better!
With regards to the original N64 version, there is a guide on the wikia,
During the final six hours of the Third Day, if Link goes to the Bomber's Hideout and looks through the telescope and exits when 00:00:02 or less time remains, the game will go to the next day instead of going to the Game Over scene. However, the game cannot be paused until Link proceeds in time by dancing with the Scarecrow. If Link dances with the Scarecrow again, Link will be taken to Dawn of a New Day, only here Link can roam freely with no end cutscene. Playing the Song of Double Time will only freeze the game, so dancing with the Scarecrow is the only known access to the Start menu. Time still flows regularly, but the Clock Tower Door will remain open.
This glitch can also be done using the telescope at the Pirates' Fortress. This glitch will not work on Wiiware.
This glitch is also reproducible on the 3DS remaster, as this post describes:
By using bottle duping over certain masks like the gibdo mask, you can screw up time so badly the game will think you have negative hours left, which means you're on 4th day. Next time it hits 6 AM, the "Dawn of a new day" screen appears, at which point the game tries to load the "credits" version of whatever map you're on, because that's what 4th day really is, the credits.