Hello,
I show this also and here is the reason.
From my side, our server is located to CET timezone (GMT+1 or +2 depending the season) and monday is the first day of the week.
On some browser (e.g. Librewolf or using some web browser extension like Disable JS) the weekly calendar widget show wrong date due to the browser of the visitor that block ajax/javascript.
There is nothing you can do except authorize JS/ajax in the browser or change the browser of the web site visitor.
It is explained in this topic :