Since there are 12 months, and 12 hours on a standard dial, it makes sense placing the 12th month at 12 o’clock.
Guessing it must have been a purely aesthetic call to place Monday at 12 o’clock, though this isn’t consistent with that spot displaying the last element of its series (or are there calendars/cultures where Monday is viewed as the “last” day of the week?)
Oh, Lange… with your renowned German sense of humor… you!