What Wiki says:

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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 moveable feast in the Christian calendar. It is moveable because it is determined by the date of Easter. In the Catholic Church, it is the Memorial of the Blessed Virgin Mary, Mother of the Church.

Whit Monday gets its English name from "Whitsunday", an English name for Pentecost, 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.

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https://en.wikipedia.org/wiki/Whit_Monday

Here it is a public holiday

First Easter Monday will come now.