Hello,
I’m reading here tips to change calendar week start day but I think there’s a bug in plugin. 
I have czech instance of WordPress. In General page in WordPress I have Monday as a week starting day. In Easy Appointments plugin setting (5. Settings / Date & Time) I have properly set calendar locale as “cs”. Tried to change to de/sk/… but no change. Despite of that my calendar weeks (in backoffice) started on Sunday. On website (customer part) it is okay, weeks in your EA form are starting on Monday (= ok).
Could you please check if the appropriate variable/setting/constant is called in calendar view in backoffice?
Here you can find screenshots.
Thanks Stepan