Juhannus was originally a pagan celebration of midsummer occurring around the summer solstice in June. It's celebrated in Sweden and the Baltic countries also. The name 'Juhannus' comes from the Christian church's appropriation of the holiday, from John the Baptist, Johannes Kastaja in Finnish, Johannes-> Juhannus. By the Church's reckoning it should be June twenty-fourth, six months from Christmas, but nowadays Midsummer's eve falls on the Friday before that. That's when people have their celebrations, even though Midsummer's day, the following Saturday, is actually Juhannus. (We also celebrate Christmas on Christmas eve, not Christmas day. Guess we Finns are just wacky that way...)
Most Finns celebrate this holiday at their seaside or lakeside summer cottages, or beg an invite from a family member or friends. Juhannus is a holiday you can spend with friends, not usually a family-oriented one, like Christmas, for example. Integral parts of the celebration are the sauna, of course, Midsummer bonfires, and dancing. And getting really, really drunk, but that's unfortunately true of most Finnish celebrations.
Juhannus is a celebration of fertility and light, so it has a lot of customs to guarantee a good harvest and luck in marriage associated with it.
For example, people used to bring birch branches and other green stuff into the house. So far, so predictable, right? But then there's the vihta (etymology from a Russian word for washing rag, or from the same family as Finnish words vihma=drizzle or vihistä=whistle), a bunch of birch branches bound together that you use to whack yourself with in the sauna. Sounds bonkers, doesn't it? It's actually not that bad. It doesn't actually hurt, and it does smell nice, and it's supposed to have beneficial effects on circulation. There are different versions of this using different kinds of branches and flowers, and a special kind of Vihta can be used for a love spell, it's said. (Sorry, couldn't find an exact 'recipe' for this:))
Other beliefs have to do with scaring away evil spirits, like lighting the bonfires, loud, rowdy behaviour, and getting drunk. (Yeah, like we Finns really need an excuse for that!)
For your amusement, here are a few Midsummer spells I translated from the list on the Finnish Wikipedia site:
For seeing your future spouse:Look into a well, lake, or spring on the night of Midsummer's eve. Preferably naked.
Place an untied vihta (see above) under your pillow. You'll dream of your hubby/wife-to-be tying it up for you.
If you're feeling industrious, pick seven different flowers from seven different fields and place them under your pillow to dream of your future spouse. (Tip: check for bugs before you put these under your pillow. Seriously. I speak from experience.)
Feeling lazy? Just sleep with the sock on your left foot inside-out.
Don't care about love? Craving wealth and power instead?
To get magical powers, you need to find a flowering fern (or other plant that doesn't usually produce flowers) on Midsummer's night, and pick it up, quick! If you happen to see a will-o'-the-wisp while your out flower-picking, there might be treasure buried under it. (Hope you brought a shovel!)
Here you go:
So, that's all folks, Happy Midsummer!
Häkkinen, Kaisa: Nykysuomen etymologinen sanakirja. Juva: WSOY, 2004.
Suomen sanojen alkuperä, Helsinki: SKS, 2000