My yard in the DC suburbs, which backs onto a wooded area, is excellent firefly viewing territory (though they're just a little bit less spectacular since houses were built just beyond the woods -- fewer trees, more light up there now). Late June - early July seems to be the prime time for them. Each year I worry that they won't come back, but they always do. It amazes me how high in the trees they fly. I love 'em -- they make the woods sparkle!