You have to get the Kafei Mask from the Mayor's wife in the right side room of the Mayor's building, then talk to Anju in the Inn just after 2pm while wearing the Kafei Mask, and Anju will ask you to meet her in the Inn's kitchen at 11:30pm (can sneak into Inn through a top door via a ddku flower). Anju will give you a letter, and put that letter in any postbox. After 4pm on the 2nd day, –c–