The Firefly Festival is held each year on the 24th of Summer. It is purely a romantic festival that serves to increase affection levels with one bachelor or bachelorette of your choice, or, if you are already married, you will attend with your husband or wife instead.
The day before the festival, talk to your special someone (if their affection level for you is three hearts or more), and they will invite you to watch the fireflies together. On the day of the festival, go to the Caramel Falls before 7:00 PM, and the event will begin. The festival ends at 10:00 PM.