This is a little late, but it depends on your relationship with the girls. As soon as you get one of the girls to 💛 💛 💛 (three) about you, then you will get a message from Flak, Nic, and Nac. You will go to their cave and they will tell you a story about the blue feather. The first time I played it happened right at the beginning of summer of year 1, and I had taken a flower to Celia every day since the game started. Remember though, the girl you give the blue feather to has to think four red hearts about you when you read her diary. To read the diaries, Celia's is located on her bed, Muffy's is on the table in the bar, and Nami's is in her room at the Inn.
Hope this helps!