Midsummer's Eve is on a Monday this year, so it'll be a short evening. But it'll be a big bonfire. I walked by the spot where five neighboring co-ops build their bonfire and they've collected a heap of wire spools.
Basically, it's light a fire and eat a hot dog. As a kid, finding the right length stick to use to roast a hot dog was paramount since those fires are hot. (We don't do marshmallows in Norway.) As a teenager, I and some friends got used tires from my grandma and rolled them down to the shore. After much struggle, we got them lit. A thick, ugly black smoke rose up on what must have been the rainiest day that summer, and we spent a while huddled under umbrellas, hoping no police boat would come by. As an adult I once dressed up as a witch to light the neighborhood bonfire. In the old days one sometimes would hang the figure of a witch over the fire. I was spared that fate and instead offered a free hot dog.
Another charming tradition is the one where you pick seven seasonal wildflowers, and put them under your pillow on Midsummer's Eve. Whoever you dream of that night will be your true love. I didn't dream the one time I tried it, so I can't tell you if it works or not. Maybe I should try again.