Apparently this is real.
On September 1752, our calender lost 11 days.
It is said that on this day, England shifted from the Roman Julian Calender to the Gregory calender (i.e. the calender we are using now).
A Julian year is 11 days longer than a Gregorian year. So the king ordered 11 days to be wiped off.
New Year was in April according to Julian date. Many people were not happy or contented to give up the old customs to celebrate New Year on April. To counter measure that, the King finally issued a royal dictum;
which stated that those who celebrated 1st April as the new year's day
would be labelled as fools. From then on, 1st April became April Fool's
How true is that? I don't know.
But the part where September 1752 had 11 days wiped off is definitely real.
Believe it or google it :P
Source: Wierd World