They show up late May to July 4th around here.

I was in a screen house tent one night down by a little stream when a rain storm came up and drenched the screen walls of the tent with flowing water. The fire flies came up close to the tent walls for shelter and when the would illuminate the watery walls of the tent magnified the light 10,000 into a great balst of light from all the lenses of water in the screen. Soonthere where hundreds of fier flies and the tent was a magnifagorium of lights patterns and rhythms.