These need to be added to the Content Service if we're to get them.

How is this done?

Can it be done for all languages?

Copy paste from:
https://www.mediawiki.org/w/index.php?title=Topic:Ulh3p09eog4w6z1y&topic_showPostId=veyjnt131crupooi#flow-post-veyjnt131crupooi

The 'onthisday' data is served via this rest endpoint. Here's an example of a call to the endpoint:
https://en.wikipedia.org/api/rest_v1/feed/onthisday/events/01/15

The endpoint extracts this data by teasing out some structure from "day pages", like https://en.wikipedia.org/wiki/January_15.

It's not a perfect approach - ideally we'll get this data from wikidata at some point (or use this endpoint to seed wikidata records for days of the year?) - but it's been useful in the mean time.

The structure of these day pages varies a bit (or a lot) by language wiki, so I tried to abstract away the language logic. Here's a mirror of the file that contains the language specific logic used by the onthisday parser (which I wrote). I added handling for a few languages which, at the time, seemed to have a page for each day of the year, but I got side-tracked by other work since.

I'd like to circle back and audit the state of all language wikis' day page coverage again. If a language has a page for every day of the year the parser isn't too hard to update iirc. I seem to remember it taking an hour or 2 per lang?

Change 776264 had a related patch set uploaded (by Matt Cleinman; author: Matt Cleinman):

[mediawiki/services/wikifeeds@master] Add In the News in for ukwiki

https://gerrit.wikimedia.org/r/776264