Where I've found this is for Euro Truck Simulator 2, when clicking the game in the librarly, at the top with the latest news. One of them is an event titled "Witches of Brackenreach Event". But this should by extension apply to all events for all games.

The issue is that this is listed as "Oct 16, 9:37 AM - Nov 17, 12:59 AM CEST", even though I set Steam to Swedish. For me, it should appear as "16 okt., 09:37 - 17 nov., 00:59 CEST", but does not.

Steam has a lot of these locale-based issues, and I would love to report them all to Steam somehow, so it can be fixed and appear correctly for everyone.