Tonight is Walpurgis night or Valborgsmässoafton as we call it here. Or Siste April (Last April) as it's known at my alma mater, Lund University. I'm not celebrating it this year and will not be watching the bonfires, but am taking the opportunity to wish all of you a pleasant evening.
Valborg is a pre-Christian festival that like many others was re-made into a Catholic day of celebrating. Thus it became part of the cult of Saint Walpurga (Valborg in Swedish). Today it's mostly seen as an informal, non-religious holiday celebrating the arrival of spring. In Sweden, the tradition to light bonfires began in the 18th Century in Uppland and have then spread to other parts of the country, though still today not all parts of the country have it. At the universities of Uppsala (in Uppland) and Lund, Siste April means many festivities and traditional outdoor university choir conserts. But though I studied in Lund for two years, I've yet to experience the Valborg spectacles at sight.
Singing is part of many of the more popular holidays. When singing on Walpurgis Night and May 1st it is said they "sjunger in våren", more or less meaning to welcome spring by singing.