[nicoburns]

Yes: a plain JSON endpoint with no authentication! It's the perfect way of exposing this kind of data.

[londons_explore]

I wonder how the backend works... Is it a microservice which queries over a globally distributed 'bank holidays' database, which has a frontend for some admin to add/remove bank holidays, and perhaps a custom approval workflow to approve/deny bank holidays, and maybe a system to make new bank holidays 'go live' on a certain date/time?

Or is it just a static file someone wrote in vim and checked into git which got hosted as a static asset?

[ZeroGravitas]

https://github.com/alphagov/calendars/blob/master/README.md

[londons_explore]

It's a static file manually written (and slightly post processed).