35978d12_Zelda___Majora_s_Mask____Songs__Where_to_play
[How to play] Inverted Song of Time [How to learn] You don't actually have to learn this song like you do most songs. Once you play the Song of Time and use it to return to the dawn of the first day, you can play this song, too. Though, if you want to be formal about it, you can go to one of the two scarecrows in Clock Town (one is in the Items shop in West Clock Town and the other one is in the observatory.) Talk to them and they will ask you if you want to learn the song. [Where to play] 
You can use this song whenever you wish. It slows down time for you by half so it would be wise to use it everytime you start back on the dawn of the first day. Though, if you are doing a quest and would like time to move at its normal pace, play it again to speed things up.
The Legend of Zelda : Majora's Mask -- Songs
Where to play