I think it's worth separating the concept of official, government-designated public/bank holidays - something around 140 years old - from that of the seasonal, traditional celebrations which have carried on in the various parts of the UK for centuries. If you look at the historical development of bank holidays below, you can see that the actual days have changed quite a bit certainly since the 1970s.

Celebrating seasonal events is not dependent on the whims of government. The First of May, for example, has had rites and traditions associated with it long before any Act designated it as a formal bank holiday - and those rites and traditions will continue in the 'rite' circles (ho ho) if the holiday is moved elsewhere.

Let's not forget that the concept of the Bank Holiday - revolutionary in its day - was to give everyone, rich and poor a formal day of rest from their everyday toil. That's the spirit of the Bank Holidays. When they occur may be inconvenient or convenient, depending on your point of view and circumstances, but they don't spring from any fountain of seasonal celebrations.

============
The 1871 Act designated four holidays in England, Wales and Ireland (then wholly part of the UK), and five in Scotland:

England, Wales and Northern Ireland
    * Easter Monday
    * Whit Monday (which could fall anywhere between 11 May and 14 June)
    * The first Monday in August (later changed to the last Monday)
    * Boxing Day (26th December)
Scotland
    * New Year's Day
    * Good Friday
    * The first Monday in May
    * The first Monday in August
    * Christmas Day

1903

St Patrick's Day (17 March) became a bank holiday for Ireland only.

1971 Act
The Whitsun bank holiday (Whit Monday) in England, Wales and Northern Ireland (which could fall anywhere between 11 May and 14 June) was formally replaced by a fixed spring holiday on the last Monday in May. The last Monday in August was formally made a bank holiday in place of the first Monday in August in England, Wales and Northern Ireland.

1973 Act
2 January became an additional bank holiday in Scotland.

1974 Act
New Year's Day became an additional bank holiday in England, Wales and Northern Ireland, and Boxing Day became an additional bank holiday in Scotland.

1978 Act
The first Monday in May in England, Wales and Northern Ireland, and the last Monday in May in Scotland, became additional bank holidays.