Let "Availability Calendars" start on monday I created this website http://peace-in-the-city.be/en/book-now and on that page you will see I used a Calendar to show the availability of the apartment. ( http://drupal.org/project/availability_calendars )

I tried to let the calendar start on Monday. I googled for the solutions and did change my regional settings local to start of the week to Monday. It worked after that for a couple days. But for some reasons it went back to Saturday. Changing the regional setting has no effect anymore now.

At this point, I'm clueless... Any tips, links and ofcourse working solutions are welcome. 

PS. I did not change anything in the rAvailability Calenda module. I'm just install modules - enable/disable - or change configuration.
As a best practice whenever an issue like this arises - disable the module, uninstall, flush all cache, verify settings for regional locale is set to start on Monday. 

Now re-enable the module. Flush all cache. Re-configure. And see if things are working as expected.

As an added additional option if you care to contribute to the community, on a fresh install of Drupal, attempt to recreate the issue that "potentially" caused this issue to arise by re-tracing any steps you may have taken (other contributed modules, other seemingly unrelated changes, etc.). If you're able to trace what caused the issue, then perhaps the potential bug can be fixed.

Of course, regardless if the uninstall, reinstall works and functionality doesn't again fail after a few days then you have solved your issue! This should be a fairly typical go-to strategy of troubleshooting for any module issue.