Majora's Mask features an internal clock that counts down for 72 hours. At the conclusion of 72 hours, the moon will crash into Termina. One game hour equals about one minute of real time, so to play through the whole three-day cycle it takes a bit longer than a real hour. Link travels back in time at the conclusion of the 72-hour period and lives the same three days over and over until he stops the moon.
When you play the Song of Time to travel back to Day 1, there are some items that you won't be able to take with you. Most items are able to make the trip through time, and once you've defeated a boss you'll win his remains and be able to warp directly to his chamber if you choose. Here is a list of what you can save and what becomes lost.
C-Button Items (Bottles, Hero's Bow, etc.)
Dungeon Maps and Compasses
Ocarina Melodies (Except for the Scarecrow's Song)
Rupees (If you deposit them in the Clock Town bank)
Great Fairy Powers
Activated Warps (Owl Statues)
Bombers' Notebook Notes
When the Gibdo says, "Leeeave me a ghost...A big one at that!" Deliver a Big Poe.
When the Gibdo says, "Leeeave me something blue that restores health!" Deliver a Blue Potion.
When the Gibdo says, "Leeeave me ten refreshing blasts!" Deliver 10 Bomb's.
When the Gibdo says, "Leeeave me something small and creepy!" Deliver a bug.
When the Gibdo says, "Leeeave me something that makes a blinding flash! Preferably ten of them." Deliver 10 Deku Nuts.
When the Gibdo says, "Leeeave me something deliciously fresh!" Deliver a fish.
When the Gibdo says, "Leeeave me something warm that can melt ice and quench one's spirits." Deliver Hot Spring Water.
When the Gibdo says, "Leeeave me something delicious to chomp on. Something that sprouts when it's watered. Preferably five of them..." Deliver 10 Magic Beans.
When the Gibdo says, "Leeeave me something rich in calcium!" Deliver some Milk.
When the Gibdo says, "Leeeave me H...2...O." Deliver Spring Water.
You must first speak to the person (or check the object) to whom you wish to give the item, then equip the item and press the appropriate C Button when prompted.
All areas and dungeons in the game will be reset to their original state, minus the major items collected, when saving. For example, if Link must complete a task in the Northern Mountain area in the springtime, he will need to defeat Goht again.
If you have the Bomber's Notebook, highlight the Bomber's Society entry with the Control Stick, then highlight any one of the entry's blue bars with an exclamation point to read the code. If you do not yet have the Bomber's Notebook, you can speak to any of the Bombers wearing a blue cap to have it repeated.
If you get mugged by Takkuri, you can simply save and reset the game or you can purchase back the stolen item at the Curiosity Shop.
No. To complete all of the side quests, though, you'll need to find all 24 Masks.
Quite simply, we don't know.