Whit Monday used to be a bank holiday, but was moveable, depending on the date of Easter. In the 1970s this was fixed as the last Monday in May and renamed Spring Bank Holiday. The school half-term was known as Whit week but again its occurrence depended on the date of Easter.
As I said, it used to be a holiday here too. The problem was that the earlier Easter fell, the earlier Whit came around and the weather had a chance to be poor, that's why it was changed to a fixed date. Didn't help the weather, though.
Apparently the Whitsun Bank holiday ceased in 1967 but the Spring Bank Holiday was not formalised till 1971. I cannot remember what happened in the intervening years. I was in the RAF at the time and we didn't get Bank Holidays.