A bank holiday is a day in which banks (and maybe other businesses) are closed. There are, however days that banks and businesses are closed that are not bank holidays: Christmas Day, Good Friday.
So people of the world if your country does not designate a special day for the banks to be off, then it is not a bank holiday!
So, why do us, the rest of the world refer to any holiday as “bank holiday”?
Short answer: Because we are ignorant.
Long answer: We are ignorant and we are too lazy to do our research. We just use words and expressions we hear around without knowing their meaning. We just assume we can figure it out from the context. Surprise: we can not! Why? because you can not possibly match the context 100%.