On the 2nd of Fall, Carter will come by in the morning and ask if you would like to participate in the Music Festival the next day. If you agree to join in, meet him at the entrance of the Church at 6pm on the evening of the 3rd. He will let you in the church where various villagers are waiting for the festivities to begin. To start the music talk to Carter again. You will be playing the Orcana, Elli and Ann have Recorders, Mary plays the Organ, and Karen is singing. There's a cute little song that plays and then the festival is over. After that you will reappear in your farm house at 6am on the 4th.