Literally White Sunday in the Old English. It is the Christian festival of Pentecost that occurs 7 Sundays after Easter Sunday. White Garments were worn on this day.
In England, Whitsun was a traditional time for Church Ales, Morris Dancing, Horse Races, Whitsun Plays and festivities of all sorts. In Gloucestershire there are traditions involving cheese - both Cheese Rolling (Randwick) and Bread and Cheese throwing (St Braivels). Cheese rolling takes place elsewhere such as Stilton in Cambridgeshire.
Whitsun marks the beginning of Whitsuntide.