Make sure you have an up-to-date backup, like daily. Some users report data loss with the sync feature, which is still in beta!
An easy way to create a backup once is to create a compressed archive of the local folder. I do that before messing around with date format, using new plugins or before updates.
That also annoyed me. I solved it with these two plugins.
If you haven’t found the solution already:
Change this in the config.
;; Specify the first day of the week.
;; Available options:
;; - integer from 0 to 6 (Monday to Sunday)
;; Default value: 6 (Sunday)
:start-of-week 0
Source: Make "First day of the week" in the date picker configurable - #10 by brsma