Night 0 (A.K.A. Night 9) is the 0th/9th night in The Return to Freddy's 3, It is a glitch night that can be followed by a step-by-step tutorial, In this night, Only Vigo is active and the cameras will crash in an early part of the night. (This also applies to the "View Animatronic" feature used to find Lockjaw)
How to unlock
Go on any night (Untested with later nights) and go on any mini-game that will send you back into the night, But don't complete it, Do the same thing you would do to hack UCN's game data, But edit TRTF 3's save file instead and change the current night to 9, Then complete the mini-game, It shall take you to night 0. (It will go to night 1's startup screen without the newspaper)
Completing it will take you back to the title screen with 4 stars and everything unlocked. (Besides the Plushies)
Night 0 in TRTF2 is renamed to Night 9, You can use any number (Untested with decimals, minus numbers, symbols, letters and ten digit numbers) into the "Level=" bar and it will appear, however, it requires the Night 8 trick to be seen and I am unable to do it nor get an image of it, so please upload it to the gallery if you succeeded.
Night 0 also appears in TRTF4, The cameras will also crash and only Lockjaw is active (also Freddy if you let the power go out.), It can be unlocked the same way as The Return to Freddy's 3, Just edit the game data.