Valborg is one of the most important festivities in Sweden, although it’s common to plenty Centro-European and Nordic countries. It is commonly known as Valpurgis Night in English speaking countries.
The festivity and its name comes from Saint Walpurga and its canonization in May 1, ca. 870. She was born un the United Kingdom, and greatly helped to evangelize the Germans.
In Uppsala, the common celebration is to go raftin in the Fyris river with funny homemade while having a light breakfast of strawberries and champagne. After that, there are gatherings in parks with music and a lot of party.
In Stockholm, probably the most famous way to celebrate it is to go to Skansen (an open air museum and zoo), to a day of concerts and live music from different choirs and orchestras of the city, and to end everything with a huge bonfire.
Well, the bonfires are not only from Skansen, and neither only from Stockholm or Sweden, as probably it’s the most common way to celebrate Walpurgis Night everywhere. Kind of similar to how the Noche de San Juan is celebrated in Spain.