Pentecost or Whitsun is observed on the seventh Sunday after Easter. Whit Sunday commemorates the coming of the Holy Spirit in the form of flames to the Apostles.
The recent adoption of a Late Spring Bank Holiday on the last Monday in May is an attempt to deal with the fact that Whitsuntide is a moveable feast, dependent on the date of Easter.
The general public still refers to this holiday as "Whit Monday." What did you do today? Any concerts in the park?