For the UK at least, there’s https://www.gov.uk/bank-holidays.json

What does "bunting" mean in the date entries?

Bunting just means decorations, the value is true when the government recommends decorating for the holiday. It's reserved for celebrations, so events like the queens funeral have it set to false.

Isn’t there black bunting?

Personally this (along with things like address lookup) are things that I consider a government should always provide. Consistent data.

Stuff like this is often officially communicated through some kind of government newsletter. Other times it's just custom and doesn't have a formal notification.