They light up the fields when they are looking for a mate, timed to when the grasses mature and go to seed. One memorable year, just past dusk - my kids and I saw them lighting up the sides of the road on Rte 109 between the highway and Meredith. It was enchanting. I hope you see them this year!
Here's a link to a Firefly page
http://www.firefly.org/firefly-habitat.html