Whitsun (Old English for "White Sunday") is the 49th day (seventh Sunday) after Easter Sunday. In the Christian calendar, it is also known as Pentecost, commemorating the descent of the Holy Spirit upon the disciples.
Pentecost is on Sunday 10th May. Whitsun is Whitsunday and is on the 24th May. Whitmonday is on 25th May, and is a Bank Holiday. In recent years the name Whitmonday was changed to Spring Bank Holiday.