At midnight on the third day the entrance to the Clock Tower will open. Proceed up the stairs to confront the Skull Kid, the Moon overhead. You can’t hurt the Skull Kid, he’s a bit out of your league at the moment.
What you need to do is what the Happy Mask Salesman said and retrieve the Ocarina of Time . This is accomplished by hitting Skull Kid with a bubble blast.
A flashback is triggered when you claim the Ocarina of Time where Link remembers Zelda playing the Song of Time . Take out the Ocarina (which is now a set of brass pipes) and play the Song of Time to return to the Dawn of the First Day.
Enter the Clock Tower and speak with the Happy Mask Salesman and he will teach you the Song of Healing . This will cure the curse that the Skull Kid placed on Link and you acquire the Deku Mask which allows you to become a Deku Scrub at will.
The Happy Mask Salesman is very upset that you have not returned Majora’s Mask, you have no choice but to acquire it before the end of the third day or else something terrible will happen.
Head back to South Clock Town and Tatl will inform you of Tael’s words about the four areas outside of town, suggesting that you start at the nearby swamp.
Note : You will receive the Bomber’s Notebook at this point. In the original game you would need to find the Bombers again or return to the Astral Observatory.