If you have a marriage candidate at 3 hearts or more, you can ask them to be your date for the festival. You will need to ask them to meet you at the festival on the 6th of Summer. Your date will then meet you at the festival for 7:00PM on the 7th - don't be late! You can only ask one date, and once you've asked somebody, you cannot change your mind. At 7PM there will be an event that plays as you and your date watch lanterns float down river. The trigger spot for the event is at the water's edge. The event will automatically end at 11PM.
If you are already married, you can meet with your family at the festival. Your spouse will bring any grown up children with them, and will meet you at 7PM for the event. You do not need to manually walk your family to the festival for the event to be seen. If you want to get a picture taken by Simon - you may want to walk with them, however. The event will end at 11PM.
There are no shops set up at the Firefly Festival. There are several villagers you can socialize with however, and Simon is available to take photos for 100g. Ruth, Phoebe, Hamilton, Perry, Mira, and Bo can be seen at the festival.