Couple's Mask Uniting Kafei and Anju is the most involved good deed you'll be asked to complete, and it involves the collection of three masks. The sequence spans all three days, and you cannot travel back in time at any point without losing your progress.
Day 1 While wearing Kafei's Mask, talk to Anju at the Stock Pot Inn between 2:18 p.m. and 9:10 p.m. Meet with her that evening at 12:00 am to receive her love letter to Kafei, then deposit it in a mailbox so the postman picks it up during his next run.
Day 2 By 3:40 p.m., Kafei will have received the letter. Visit Kafei in the Curiosity Shop back room by entering the door at the Laundry Pool between 4:15 p.m. and 10:00 p.m.
Day 2 Inside the back room of the Curiosity Shop, Kafei will explain his troubles and give you the Pendant of Memories. Return to the Stock Pot Inn to deliver this symbol of undying love to Anju.
Day 3 Receive the Keaton Mask and talk to the Curiosity Shop owner in the back room to receive the letter, Between 1:00 p.m. and 10:00 p.m. Before delivering the letter soar to ikana canyon and jump of the ledge and go to the Hideout.
Day 3 Between 6:00 p.m. and 7:00 p.m., Sakon will enter his Ikana hideout. Hide with Kafei behind the rocks when Sakon enters, then sneak in after him. After helping Kafei reclaim his Sun's Mask(YOU SHOULD HAVE SIX MINUTES LEFT TILL MOON TIME), Deliver the letter to the postman in his office at night, then speak to him at the bottom of the moomoo bar, after he's delivered it (IT WILL BE VERY CLOSE TIMING BUT DONT WORRY). Return to the Stock Pot Inn and go up to the second floor of the inn and go into the first door you see and you will be in a small room with anju. Wait there and talk to them after kafie arrives to complete your complicated task and receive your reward. You Still have time now play the song of time to keep the mask and save and quit if you don't there is no save point on the moon making it impossible to save or continue. If you go to the moon before saving and returning to first day you will have to do it all over again.