In the UK, Parliament closed last Thursday for the Whitsun recess, but, strictly speaking, today is not Whit Sunday. Whit Sunday is a movable holiday; it is the seventh Sunday after Easter (which means next Sunday, 31 May, this year). It is a religious festival and commemorates the descent of the Holy Spirit on the day of Pentecost. Pentecost is another word for Whitsun or Whit Sunday and derives from the Greek, meaning fiftieth day - the seven weeks after Easter.
Whit Sunday means white Sunday in Old English and Old Norse. The word white probably refers to the custom of the newly baptised wearing white baptismal robes. Whitsun was always a jolly festival - it was probably seen as the start of summer - and people traditionally wore new clothes and took part in all sorts of merrymaking. Morris dancing is associated with this period, as is the custom of well-dressing (ie the decorating of wells) in some areas.
The Whitsun holiday was replaced in the early 1970s by the fixed Spring Bank Holiday - always the last Monday in May.