The onthisday scraping logic for frwiki seems to miss certain types of events.
Compare the output of onthisday:
https://fr.wikipedia.org/api/rest_v1/feed/onthisday/events/12/26
...with the wiki page itself for that day:
https://fr.wikipedia.org/wiki/26_d%C3%A9cembre
The wiki page has noticeably more events. It's likely because some of the events are in doubly-indented bullet points. Also, in addition to generic Events ("Événements"), the page has other headings like "Art"/"Science" etc. that are not getting parsed.