There is often a difference of 11 to 13 days between the date an event occurred in history and the date it is celebrated on the calendar today. This is due to a discrepancy that occurred in the shift from the Julian to the Gregorian Calendar.
The Julian Calendar, named for Julius Caesar, called for an extra day (Leap Day) to be inserted once every four years, in order to keep consistent with the 365 & 1/4 day solar year.
