Heimskringla does not include Midsummer among the three required blóts it lists. But then in the Saga of Ólafr Tryggvason the king promises to meet with the bönder at the miðsumarsblót. So there could be one.
One suspects there always was one, because with Yule, Midsummer is the festival that has persisted most strongly in folk tradition in Scandinavia and Germany alike. (It’s also an important national holiday: in Finland it coincides with the national Flag Day; in Denmark, a patriotic song is sung; in Sweden in particular, it marks the traditional start of vacation season.) In Scandinavia, it’s now celebrated on the third weekend in June; in Germany it remains associated with St. John’s Day (Johannistag), June 24th, where the medieval church relocated it (one of the sagas actually notes that that is what Ólafr did.) There are also a surprising number of English folk traditions.
There are two strands to the Germanic folk celebration of Midsummer. One is flowers. Swedes, Finns, and Germans dance around flower-garlanded “maypoles”; at Midsummer. English villagers decorated houses and wells with flowers gathered on Midsummer Eve, and English girls had various options that evening for divining who they would marry; one was to scatter fern seed on the ground and then look back over their shoulders (but I seem to recall that ferns have no seeds); another was to rise at dawn and wash their faces in a basin of water in which his face would be reflected. Swedish girls pick seven different flowers on the way home from the dancing and sleep with them under their pillows that night in order to dream of him. Germans gather a bundle of herbs and flowers – regional tradition prescribes which ones, and there may be 9 as in the Anglo-Saxon Nine Worts Galdor or as many as 99.
The other is fire. In Finland, where folk traditions are a mix of Nordic and Finno-Ugric, bonfires are lit to scare off evil spirits. The Danes, Norwegians, Germans, and southern Swedes also light Midsummer fires; in Denmark a witch is ritually burned; in Germany, especially southern Germany, all the young people were expected to participate in building the Johannisfeuer, and more important than the dancing was that couples jumped through the flames together for good luck and health in the coming year, and after it burned down to ashes, the stock was driven over the coals for its good health. The Johannisfeuer was traditionally a needfire-kindled on the spot by purely mechanical means-and was built on high ground. The Scandinavian fires, in contrast, are traditionally on a beach or at lakeside.
In the British Isles, where the Midsummer fire traditions are now largely forgotten, they were clearly related to fertility: bonfires were thought necessary to the success of the apple crop, and farmers in parts of England and Scotland carried fire around their fields every year at Midsummer. In Ireland, burning brands were thrown into the potato fields. In some places, the seeds for the next year were mixed with the ashes from the bonfire, and all households rekindled their fire from it as was done at Yule or New Year’s elsewhere. Like May Day, this was a time for morris dancing, in particular groups known as the “marching watch” that went from bonfire to bonfire carrying cressets (lanterns on long poles), with people dressed as a unicorn, a dragon, and six hobby-horse riders.
In some places, including in Devon until the 1950s, flaming wheels were rolled down hillsides; the continuing tradition in Gloucestershire of chasing a wheel of cheese down a breakneck slope at Whitsun may be a modern adaptation of this.
As at Eostre/Ostara, many modern heathens try to stay up all night on Midsummer Eve, the shortest night of the year. The bonfire tradition, with dancing, leaping, and love divination, suggests that was part of the tradition, and there are a wealth of stories about the magic of “St. John’s Eve,” including of course Shakespeare’s A Midsummer Night’s Dream. Bonfires, fire processions, decorating with flowers, and gathering herbs are probably also ancient traditions.
But if there was traditionally a blót at this time, it has left almost no trace. It has been argued that there were four seasonal blóts, as in other European religious traditions; but there is no Midsummer Thing tide. So perhaps Midsummer blót was local, in the villages and on the farms.
Many heathens, and non-heathen pagans, call this festival Liþa (Litha), because that is the corresponding Anglo-Saxon month name. But unlike Eostre, there is no mention of a goddess after whom the month is named. There is UPI that there was such a goddess, but her name was not Liþa.