@Vincent BergI'm a Latvian. We are keepers of a little know quasi-religious borderline matriarchal worldview that long ago may have been popular throughout much of central Europe, from northern Italy to nearly Urals, among settled peasants -- as opposed to nomadic or semi-nomadic horse pastors who often overran parts of the same lands but do represent very different cultures.
After such an introduction, back to bonfires and naked dancing. There's the Midsummers Night. A short, but intense festivities, and the supposed New Year's day... maybe. Short, for Summer Solstice being in an extremely busy part of the year for most peasants, and, where I live, that shortest night is really as short as only six hours sunset to dawn, and yes, it only barely gets dark as you can see Sun's glow in the clouds coming from due north, She's being just barely under the horizon...
...so it may seem to be extremely lazy to declare a religious need to maintain a bonfire sunset to dawn this exact night. Night that is, somehow, for all intends and purposes magical, and exempt from normal rules.
Including, supposedly, temporary abolishing any marriage vows (in some arguably overly extreme interpretations, any relationship guidelines whatsoever, so according to those even incest is allowed), as the song says, "Midsummer night [I] couldn't determine who was wife who was daughter, all they wore flower crowns." (Flower crown is somewhat synonymous with virginity, while wife and daughter are two words that together inclusively describe any female who isn't [your] mother ("wife" = someone's wife, any adult female; "daughter" = a girl, any unmarried woman, but also maid and sometimes whore)).
So, yes, it's a night of free love, in search of the mystical Ferns Flower that only booms this one night and could only be seen by couples (no, ferns don't bloom in the sense we usually interpret flowers, if you need a botanical reminder). As the song has it, "who falls asleep the Midsummer night, [is a lazy ass who] will sleep the whole summer." More carefully reading it, it's not hard to determine it's about sleeping *alone* not necessarily about sleep as such. Indeed, it turns out, it's borderline shameful NOT to have sex Midsummers night... yet, that's not something ever said aloud in direct words, but it's implied.
What's said, "Girls who want to be beautiful must batch in morning dew at sunrise," (naked of course, as most of magic requires, especially female magic (I think it's both for a titillating factor in the magic formulas as well as an added barrier, and also, especially in cases like this, facilitate the observable effect on, well, observers)).
That's theory. And I would include any forms of jumping over the bonfire, naked or not, in mostly pure theory as well, although there's surely someone who has done just that, possibly even last year, or at least has a friend who claims knowing someone who did. For one, the average Midsummer night bonfire is way too big to jump over. And speaking about lost traditional ways, there's a traditional way to have the Midsummers fire in a barrel on top of a long pole, referred to by a world meaning "dick" for those who knows, and setting it up was a major collective affair, so yes, "raising the dick" was one of the opening activities for the night.
Actual modern world practices vary, but outside of wild legends are usually much more tame. Although official naked runs are part of modern tradition in several places, but the actual participants are almost exclusively men. Skinny dipping at night or at dawn was still quite popular "secret" part of the festivities as recently as late last century.
Some dancing is borderline a must, still. Large outdoor concerts are commonplace.