New Calendar add-on adaptor

With plenty of assistance and goodwill from @mrstegeman, I published the Calendar add-on / adaptor.
This manages the working week, holidays and other events so that, for example, rules will not trigger a morning alarm on a public or personal holiday. With version 0.1.2, this provides integration with the Calendarific API to automatically download public holidays for the current and next calendar year.
I intend to add at least one further holiday API provider (please tell me if you know a good one).
I do think that the UI aspects could use some review from someone that is not me.

Sounds great! What countries are supported? Every country has different holidays. Is it regional? I assume Calendarific take care of that?

I tested with sg and with us-ny and both worked as far as I can tell. I will integrate at least one additional free api provider when I get time to do some research.

Of course, the main part of the add on does not care, it will support whatever you input for public holidays, personal holidays, or other events.

I documented what this addon does at https://github.com/chas-iot/calendar#readme
I would be grateful for a review of the UI, both the Thing and the configuration, as I know that I have blindspots.

I now think this addon is ‘complete’.
I added a 3rd very different holiday API provider and enhanced events to allow recurring annual events such as birthdays.