Wrong Day Name on my Page

I’m having a strange situation on my page, first of all, you might have a strange layout before hitting “shift+f5” then, the start is correct..

You see “Mittwoch” as the day (which is today – Wednesday) then you might click on the 7th

Now it shows Freitag (Friday) It should say Samstag ( Saturday ) and below in the date-line it’s correct (“Sa” above the 7)

https://cdn.livechat-static.com/api/file/v2/lc/att-old/8801096/888da724a4df5a52cb5eef0d313339c5/Bildschirmfoto%202019-12-04%20um%2022.34.11.png

Can you help me?