where did bank hollidays come from and why?

Answer 1

From the holy days of the church.
Days like Christmas and Easter all people would celebrate, often for two days even.
So the banks and other businesses would be closed the Monday after a main celebration.
As less people went to church those days, they became a free day. And in Britain they have been detached from the religious days they used to be on.
The May bank holiday is still called Whitsun Bank holiday by some, even when it is not on the Monday after Whitsunday.
And when they decided to detach the days from the religious dates, they also spread them better over the year.

Many countries still have the bank holidays on the dates the church have a celebration, and a few on the important days for the country, like remembering the end of a war or independence day.