Yes, SO true that cats invariably get ill on Friday evening of a bank holiday weekend.
But yes, have had that happen with multiple (indoor-outdoor) cats, all at once. This was a number of years ago. Turned out that one of the cats had caught dragged home a young possum that had already been poisoned, and left it under the front stairs where we didn't see it. In the course of a couple of days, we guess all four cats... at one time or another... licked or chewed on the carcass just enough to get "somewhat ill" but not seriously ill... including going off their food.
So if something like that is possible where you are, that might be one possible explanation.
=^..^=
I did consider that but I think it's more likely to have been a virus or similar