May Day is basically another name for Labor Day and occurs every May 1rst. It celebrates Spring and the hard working citizens.
This is a public holiday in Germany where most post offices and other public jobs are closed. Many people in Germany observe the night between April 30 and May 1 as Witches night where bonfires are lit and some young people play pranks.
The night of April 30th used to be seen as a witch night in old day Germany where the witch prisons would light bonfires and do wild dances in order to keep the witches out. The day was made a combined holiday with labor day to support workers rights and witch traditions.