Happy May Day!

May Day is related to the Celtic festival of Beltane and the Germanic festival of Walpurgis Night.

May Day falls exactly half a year from November 1, another cross-quarter day which is also associated with various northern European pagan and the year in the Northern hemisphere, and it has traditionally been an occasion for popular and often raucous celebrations.

As Europe became Christianized, the pagan holidays lost their religious character and either changed into popular secular celebrations, as with May Day, or were merged with or replaced by new Christian holidays as with Christmas, Easter, Pentecost and All Saint's Day. In the twentieth century, many neopagans began reconstructing the old traditions and celebrating May Day as a pagan religious festival again.

The earliest May Day celebrations appeared in pre-Christian times, with the festival of Flora, the Roman goddess of flowers, and the Walpurgis Night celebrations of the Germanic countries. It is also associated with the Gaelic Beltane. Many pagan celebrations were abandoned or Christianized during the process of conversion in Europe. A more secular version of May Day continues to be observed in Europe and America. In this form, May Day may be best known for its tradition of dancing the maypole dance and crowning of the Queen of the May. Various Neopagan groups celebrate reconstructed (to varying degrees) versions of these customs on May 1st.

The day was a traditional summer holiday in many pre-Christian European pagan cultures. While February 1 was the first day of Spring, May 1 was the first day of summer; hence, the summer solstice on June 25 (now June 21) was Midsummer. In the Roman Catholic tradition, May is observed as Mary's month, and in these circles May Day is usually a celebration of the Blessed Virgin Mary. In this connection, in works of art, school skits, and so forth, Mary's head will often be adorned with flowers in a May crowning. Fading in popularity since the late 20th century is the giving of "May baskets," small baskets of sweets and/or flowers, usually left anonymously on neighbours' doorsteps. . . .

Roodmas was a Christian Mass celebrated in England at midnight on May 1.  

Traditional British May Day rites and celebrations include Morris dancing, crowning a May Queen and celebrations involving a Maypole. Much of this tradition derives from the pagan Anglo-Saxon customs held during "Þrimilci-mōnaþ" (the Old English name for the month of May meaning Month of Three Milkings) along with many Celtic traditions.

From Wikipedia.

Walpurgis Night is apparently a major celebration in Sweden and Finland somewhat resembling New Year's Eve with something of the mirth of April Fool's Day thrown in as well. 

Saint Walpurga was a nun, abbess and Christian missionary from Devon, England to Germanic Europe from the 8th century, and "she is often called the first female author of both England and Germany." Unlike many Christian saints she was not a martyr. She is "the patron saint of those suffering from rabies," her relics were associated with healing oils, and she has been conflated in some iconography with symbols associated with the pagan Grain Mother goddess. The Saint's Day of May 1st was assigned to her when she was canonized in 870 CE.

Walpurgis Night is sometimes call the Witches' sabbath.