Searching for the way to make Mac OS X/BSD version of cal output calendars formatted for the weeks starting on monday, I've found this piece of trivia on Debian's man page

The Gregorian Reformation is assumed to have occurred in 1752 on the 3rd of September. By this time, most countries had recognized the reformation (although a few did not recognize it until the early 1900's.) Ten days following that date were eliminated by the reformation, so the calendar for that month is a bit unusual.

And indeed it is! You can check for yourself - ten days 'tween 2nd and 14th of September are, like, gone:

          $ cal -y 9 1752 
September 1752
 S  M Tu  W Th  F  S
       1  2 14 15 16
17 18 19 20 21 22 23
24 25 26 27 28 29 30