Each service provider's profile has a section which includes two calendars: One for Exceptional Working Days, a second for Holidays. In both cases when the calendars load they show with each day of the week represented as: Su Mo Tu We Th Fr Sa. Also the calendar's first day of the week is Sunday. However clicking into either of the calendars changes it's view to have Monday as the first day of the week and relabeling the days of the week to: M T W T F S S.
I can replicate this every time. Using the latest version of Appointments+ (2.3.1) with WordPress 4.9.6 on PHP 7. I have already done conflict debugging and disabled all plugins and used the 2017 theme. Problem still persisted.