Your booking calendars show Monday as the first day of the week.
Why this happens. Your site reads its first day of the week from a single WordPress setting.
How to change it. Go to Settings, then General, and change Week Starts On to Sunday. This updates every calendar on your site at once, including the search bar date picker.
If a calendar still shows Monday after you save, your browser is holding a cached copy of the page. A hard refresh clears it.