This is quite interesting:

erika@lpdlnx00 proc $ cal 9 1752
   September 1752
Su Mo Tu We Th Fr Sa
       1  2 14 15 16
17 18 19 20 21 22 23
24 25 26 27 28 29 30

...for the uninitiated, that is the September calendar from 1752. Notice that there are a few missing days :-)

No, the GNU "cal" program doesn't have a bug in it. September 1752 was when Great Britain replaced the Julian calendar with the Gregorian calendar. When that transition occurred, there was an eleven-day offset between the two calendars, so "officially", the date skipped from September 2nd to the 14th.

I thought that was interesting...