After recovering The Sun's Mask, warp back to Clock Town and go up the first room door in the Stock Pot Inn where you'll find Anju waiting. Kafei will just barely make it and reunite with his now wife, Anju. They will give you the Couple's Mask. Grab the silver rupee in the chest, deposit it in the bank and rewind time. Show the Couple's Mask to the Mayor at 10am and you'll earn a Piece of Heart.