The Julian Calendar, so called because it was adopted at the command of that famous general and demagogue Gaius Julius Caesar, was used by Europeans until the 16th Century, when Pope Gregory the Great promulgated a new calendar, aptly called the Gregorian Calendar.
Unlike the Egyptian calendar, which had 365 days in a year, the Julian Calendar had an extra day every 4 years. Unfortunately, a year does not consist of exactly 365.25 days, but approximately eleven minutes less. By the 16th Century, it was obvious that Christmas would eventually be the Spring, and would at some future time be on the same day as Easter.
Pope Gregory decided not to turn the calendar all the way back to Caesar's time, but rather to reset it to where it was in the third Century. Accordingly, 10 days were snipped from the year to make things right.
The Protestant and Orthodox countries refused to go along at first. But, seeing the necessity of the change, they eventually gave in. The United Kingdom (which included Virginia at that time) jumped to the Gregorian in 1752, as you can plainly see by executing the Unix command `cal 1752`. Eleven days were deleted from September, and George Washington, who was born on February 11, 1732, recalculated his birthday, using the correct Gregorian date, February 22.
Therefore, we celebrate George Washington's birthday on February 22, but it was February 11 when he was born.