Sorry, this is a riddle to me so I'm asking the question here -I can't find anywhere else where it could go! It's this: When was there a month with less than 28 days?
Just now I have been reading all the old questions out of interest.
The answer to your question is September 1582.
This is the month when the correction of old calendar to the new (Gregorian) calendar was made. September 3rd was followed by the date September 14.
The people cried "give us our eleven days".