In The Legend of Zelda: Majora's Mask, gameplay spans three in-game days; however, completing everything necessary within just those three days is impossible, and so one must regularly play the Song of Time to time-travel back to the beginning of the three-day cycle. In doing so, some progress is kept, but many other things are reset.
What is kept and what is lost when using the Song of Time to return to the beginning of the three-day cycle?