Happy Solstice!
Here is an interesting article about mid-summer celebrations from around the world from CNN. Here is a list of books from Goodreads about the summer solstice. And here is a romance about it.
Summer Solstice, the longest day of the year, in Oakvale is celebrated by a large party held in the meadow next to the (hopefully running) stream. There are three-legged races and backwards mule riding contests. There are apple pies and other pastries to be eaten. Cider, cyser, and beer are offered, along with a more potent alcohol called applejerk.
Gestin and Jeslynn hooked up at a summer solstice celebration and were married in the fall of that same year.

I hope your longest day of the year is a happy one.