Tonight, April 30th is Walpurgis Night or Walpurgisnacht.
The traditions surrounding Walpurgisnacht come primarily from Central and Northern European countries, and by most accounts predate the Christianizing of Europe by hundreds if not thousands of years. The modern name Walpurgis Night comes from the feast day of a nun, Saint Walpurga of 7th century Germany, and from the tradition of the Roman Catholic church of Christianizing pagan festivals.
Interestingly, the traditions of Walpurgisnacht are very similar to the later Celtic festival of Samhain, or Summers' End. The veil between the world thins, and giant bonfires are lit to drive off evil spirits and protect the living from mischief and mayhem. Pranks are common and drinking is encouraged :D
In Sweden Walpurgisnacht is called Valborgsmässoafton or Valborg, and it is one of the main festival holidays next to Yule and Midsummer.
In Germany, Walpurgisnacht (or Hexennacht, meaning Witches' Night), is the night from April 30 to May 1, is the night when allegedly the witches hold a large celebration on the Blocksberg and await the arrival of Spring.
In Estonia, there is also a strong association with witches, and some people dress up as witches on this night and wander the streets.
Finally, in the Norse traditions, this is the night where Odin received the runes.
So get out your best Witchy gear, start a bonfire and read some runes this evening! Remember that the veil between the worlds is thin, and that divination and scrying will be powerful ways to access information. Thank your ancestors and guides by pouring out a libation of mead or other beverage, and above all, have fun!
(Information source for some of this article: Wikipedia article on Walpurgis Night)