A little info on Swedish midsummer traditions
The Swedish midsommar is a very old tradition, akin to other Midsummer celebrations throughout Europe. It is one of the most important holidays in Sweden, or one of the worst ones, depending on how cynical you are. I thought I'd make a little write-up on it prior to the festivities on Friday. :)
When is it?
Prior to the 1920s, the holiday was called Johannes Döparens dag - meaning the Nativity of St John the Baptist. As such, it always occurred on June 24th, since Mary's immaculate conception took place "[i]n the sixth month of Elizabeth's pregnancy" (Luke 1:26) and John and Jesus were hence born half a year apart. In 1952, this was changed to the Saturday closest to the 24th, i.e. between June 20th and June 26th.
Note however that we typically celebrate holidays on the eves in Sweden. Nobody cares about Christmas Day here, it's all about julafton - Christmas Eve! And the same goes for midsommarafton, which is celebrated on the Friday the day before. It is not a public holiday, but the three major eves are by law equivalent to public holidays - midsommarafton, julafton, and nyårsafton (New Year's Eve).
But I thought it wasn't a Christian holiday?
This is largely correct. Although it has importance to the church, it is mostly secular. St John's day has been celebrated since about the 5th century, but many Christian holidays were deliberately chosen to coincide with earlier important dates, and June 21st is the summer solstice. The Nordic countries also celebrated the beginning of each kvartal (quarter of the year), and even though this happens about a month later, the new Midsummer tradition gradually came to incorporate both.
So how is it celebrated?
We prance around a huge phallus pretending to be frogs.
Well, perhaps not exactly. The midsommarstång is a maypole, frequently but incorrectly thought in Sweden to be a remnant of pagan fertility rites. But the pole came to Sweden only during the middle ages, and the idea of it being phallic started during the Freudian craze of the late 19th century, when absolutely everything was psychoanalysed.
The pole is clad in leaves, preferrably from birch, and with as many flowers in it as you want. The most common shape is a cross with two wreaths and two supporting beams. And the wreaths are used as crowns as well - the midsommarkrans (word not related) is part of the archetypical image of Sweden: blonde children with flowers in their hair. Here's the first hit I got from Google (image from Wikimedia, I've resized it):
As in the rest of Europe, there's a lot of dancing going on around the pole. These have almost entirely grown to become silly and somewhat childish, and are carried on as much for the sake of the kids as for tradition. One common and typical song-and-dance is Små grodorna - to the vast amusement (and besumement!) of any visiting non-natives. You can see it on Youtube here. (And for those of you whose Swedish is getting quite good, this quote from someone who came to Sweden as a child sums it up well.)
The summer solstice was a major event in folklore absolutely everywhere, and in Sweden, too. Our midsummer traditions are filled to the brim with plants and flowers - we put them in our maypoles, in our wreaths, in our hair, and in plenty of other places.
One very common tradition is to pluck seven different kinds of flowers. According to the superstition, you should pluck them on seven different fields, passing seven different gates during the way - a considerably easier feat back when people lived near small fields with gates. And you must not utter a word during this process. If you clear the requirements, you can then put the flowers under your pillow, and the next time you sleep, you will dream of your future spouse.
Medicinal plants were extra powerful if harvested during the midsummer night, and they were often dried and kept in wreaths, brooms, and bouquets, in order to preserve their magical healing properties until next midsummer.
Oh look, more Swedish fish
No holiday is complete without its food!
Unsurprisingly, pickled herring is a mainstay during midsommar, especially matjessill (from Dutch maatjesharing - "virgin herring") which is a specific pickling.
Other important foods are mostly in-season local delicacies, like strawberries, and like early potatoes that have not yet grown to full size. These are usually cooked with fresh dill and served with sourcream and newly cut chives. Rhubarb is common in deserts, if alternatives are offered in addition to the strawberries.
A lot of adults also drink snaps - a small, meal-side alcoholic shot, often flavoured with e.g. elder, blackthorn or St John's wort. Unfortunately, midsommar is frequently used as an excuse to drink excessively (mostly beer), and it is a very hectic holiday for police and EMTs. I don't mean that as a discouragement of the whole thing - most people just drink a little if at all, and it is a generally very family-friendly holiday.
Regardless of whether you drink alcohol or not, you might enjoy learning some snapsvisor - drinking songs that you sing just before partaking. Here's a video of Will Ferrell, who is married to a Swede, objectively explaining our Christmas traditions and singing Helan går - the most famous snapsvisa.
Midsummer traditions look largely the same throughout Sweden, but there are certainly regional variants. I've also not covered e.g. how it's celebrated in the Swedish-speaking parts of Finland, mostly because I have no idea.
If you do get a chance to visit our midsommarfirande, take it! We usually get good weather then, at least. :)