The Fyr Ball festival takes place during one weekend in the summer in downtown Ephraim, in Door County, usually during June.
Even though the festival is scheduled thursday-saturday usually, really things are only happening on Saturday. Friday night there was a "concert" scheduled, but it was really just some classical musicians playing inside a building. It didn't look too exciting.
On Saturday there are a lot of activities...you should be able to find a full schedule online when it gets closer. There was a boat regatta, pet parade (we didn't attend these) plus some other stuff. Also, there are craft booths set up all day, as well as a used book sale outside the library, and door prizes and a silent auction.
In the evening, there is a "fish boil" which is a big door county tradition. You can look it up online for more info. We were a little disappointed with it, however. Tickets were $16 a person, and included the fish, a few side items, and a piece of cherry pie. The fish still has bones, so is kind of hard to eat. We didn't feel it was worth $16. Also, make sure to get in line as soon as it opens, they can only cook so much fish at a time and people had to wait a long time.
At night is when things get good. They put large piles of wood all around the harbor and light them when it gets dark. Then they set off fireworks, which was a pretty impressive display.
Overall, I'd say stop by the fest in the afteroon to take a look at the crafts and buy some raffle tickets, then come back at night to see the fires and fireworks...don't plan on spending a whole day here. The fest is fun, and you should definitely check it out if you're in town that weekend, but I wouldn't go out of my way to plan my trip around it.