In Heimskringla, in “Ynglinga Saga,” we hear that the Norwegians had three mandatory blóts:
þá skyldi blóta í móti vetri til árs, enn at miðjum vetri blóta til gróðrar, it þriðja at sumri, þat var sigrblót “At the beginning of winter blót should be made for a good year, and in the middle of winter blót should be made for a good crop, and the third sacrifice should be when summer came—that was the blót for victory.”
Determining when that third blót took place is hard, since the Norse divided the year into only two seasons, summer and winter. It is commonly associated with Eostre/Ostara. But bearing in mind the difficulty of travel either to a great blót or to attack an enemy in the mud of the spring thaw, I would imagine it took place later than the early spring agricultural rites, in fact at the end of the thaw, i.e.: in late spring or early summer by our reckoning.
In the Scandinavian climate, that would put it close to May Day, which is the traditional Celtic holiday of Belta(i)ne and has been celebrated in England and other parts of Europe since the Middle Ages, but seems to have left no trace in Germanic sources.
Instead, what we have is a German folk tradition that on May Eve, Walpurgisnacht—the eve of the day of St. Walburga—the witches ride to revel together on mountaintops, particularly on the Brocken, a peak in the Harz. Walburga’s name is impeccably Germanic; it would mean “slaughter protection,” and although she was a real 8th century abbess, she was canonized after healing oil supposedly flowed from her tombstone on May 1 (which therefore became her day). She is honored in parts of Germany that preserved the memory of Holda, Perht/Bertha, and Nehalennia. One of her symbols is a dog; a dog appears on several of the Roman altars to Nehalennia, but there is nothing about dogs in the abbess’ life story. And she is said to be chased by the Wild Hunt, while in Scandinavia the Wild Hunt chases a Huldre. And Walburga’s abbey was called Heidenheimer, “heathen home.” So—following in the footsteps of Jacob Grimm–many heathens have identified St. Walburga with Holda (and/or Perht/Bertha), or seen the legend of the witches’ revels as based on memories and rumors of heathen rites. There is an excellent essay by Winifred Hodges setting forth the argument. The counter-argument would be that there is no evidence of an ancient Germanic May Day festival; the Wild Hunt is associated with winter, particularly Yuletide, and it is in winter that Holda and Perht are said to lead it. Holda is said to produce snow—not May blossoms—by shaking her feather bed, and the name Perht may be a mangling of “Epiphany.” On the other hand, Belta(i)ne was and is an important Celtic festival, specifically a fire festival. What the Swedes do for Valborgsmässoafton is very much a fire festival, and is claimed to go back to the Middle Ages.
So perhaps there was such a Germanic tradition, but it has been effaced in our written sources behind the agricultural spring rites and the sigrblót. What we have in English tradition is very different:
greeting the dawn with song, dancing around the Maypole, Morris dancing, crowning a King and Queen of the May, and other fertility rites. Alissa Sorenson describes the May dancing traditions in a heathen context. More pictures of enthusiastic continuation of the English traditions can be found here and elsewhere on the net.
So Walpurgisnacht, or Walburga as it is often heathenized, and May Day are a double feast day, and the interpretation of many scholars and many modern heathens is that it represents the conquest of the spirit of the spring over the old year—the last gasp of winter giving way to the onset of summer and new life. It is easy to associate the maypole with Freyr/Fréa/Frô, and dancing with ribbons until someone, or preferably a couple, gets bound to the pole is fun. Some stay up all night with a bonfire on Walpurgisnacht as the Swedes do, rather than doing so for Eostre/Ostara, when it is more likely to be chilly.