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Whit Monday or Pentecost Monday (also known as Monday of the Holy Spirit) is the holiday celebrated the day after Pentecost, a movable feast in the Christian calendar. It is movable because it is determined by the date of Easter.
Whit Monday gets its English name for following 'Whitsun', the day that became one of the three baptismal seasons. The origin of the name 'Whit Sunday' is generally attributed to the white garments formerly worn by those newly baptized on this feast.