As explained on the French forum, you seems to be configured for English, so may be your country is US ?
In US the 09/06/2021 is a day OFF : Labor Day in the United States
On the other hand, I noticed that Saturday is counted as an open day, while my working days setting is set to 1-5 while the first day of the week is set to Monday:
I am currently using version 15.0.3. I also tested version 16.0.0 also facing the same issue.
By the way I am from Malaysia.
Set Working day 1-5
First day of the week Monday