(Image credit: Fiendfall)
The evening of April 30 through May first is a confluence of many holidays celebrated traditionally in Europe. In the United States, it has been celebrated as Half-o-Ween in recent years by Halloween fans because it marks the halfway point in the calendar between Halloweens. That point in the calendar has been important in many cultures for as long as anyone knows.
The cardinal points in the year that do not depend on culture are the solstices and the two equinoxes. In pagan Ireland, these points are celebrated, but also the midways points between them: Samhain, Imbolc, Lughnasadh, and Beltane. Beltane on May 1 traditionally marks the beginning of summer, when the livestock can be taken out to pasture. While modern celebrations of Beltane are usually called May Day, Celtic neopagans and Wiccans have brought back the traditional Beltane bonfires, once used to purify and bless the livestock as they begin their summer outside. The image above is of one such bonfire, dressed up with a wicker man from old Druid tales. The same day is celebrated as Calan Mai in Wales.
(Image credit: Chmee2)
In continental Europe, the holiday is called Walpurgis Night, the eve of the feast day of St. Walpurga. Walpurga's official feast day is February 25 on the Catholic calendar, but was transferred to May first in Finland, Sweden, and Bavaria to commemorate the date of her canonization. It also coincided with older festivals like Hexennacht, and so follows the church's tradition of co-opting pagan holidays. Walpurgis was renowned for protecting folks against witchcraft, so the evening before Walpurgis Day, April 30th, is a night for bonfires to purge a community of witches and their evil spells. However, the custom of symbolically burning witches on April 30th is a much older tradition, like the bonfire for Pálení čarodějnic (burning of the witches), pictured above in the Czech Republic.
Celebrations and rituals of Walpurgis Night take on different flavors in different parts of Europe. They also incorporate celebrations of International Workers Day and May Day. In northern European countries, students getting ready to graduate from high school or college use it as a day of traditional celebrations that revolve around drinking and pranks, to generally blow off steam before they step into adult life.
In the US, the day is marked as Half-o-Ween, which is just a day to pretend it's Halloween because we want to. Celebrate any way you like, but if you want to incorporate the European traditions, a bonfire is their most common ritual.