Let "Availability Calendars" start on monday
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.