What is the tiny yellow moon(?) symbol on calendar?

Since the forced patching I'm seeing a small glowing yellow round symbol on the calendar icon in the main UI. I click it to check, but I find nothing special in there and don't really see what the yellow symbol (sun or full moon?) is trying to tell me. I think it is thee every time I enter a new household, then I will need to open the calendar just to make the symbol go away.

I think there must be something I miss. What is the purpose of the new feature (?). I already see the moon phase next to the weather, so I suppose it is not to tell me when here is a full moon, besides there was no full moon when I checked.

I don't have Werewolves pack, btw.

I wish the yellow symbol was not there because it only gets annoying... Most new features might give a hint like this only once, but this seems to last forever...