George Washington’s birthday is observed on the third Monday in February. Since the third Monday usually falls between Lincoln’s and Washington’s birthdays, many people celebrate it as Presidents’ Day. But the federal holiday’s name was never officially changed. It is still ‘Washington’s Birthday Observed’ even if it is commonly referred to as ‘Presidents Day’ by state legislatures and mattress stores alike.

(1700s Union Jack; early 1776 colonial flag; 1777 U.S. flag)
So when was George Washington actually born?
If you said February 22 like your calendar claims, you’re wrong.
George was born on February 11, 1732, not the 22nd.
In 1732 the colonies, under British rule, still used a form of the Julian Calendar.
Under the British system the calendar year ended on March 24 and the new one began on March 25. Thus March 24, 1732 would be followed by March 25, 1733.* (A modern equivalent would be if the U.S. calendar followed the tax year—if April 15 marked the end of 2008 and April 16 marked the start of 2009.)
Why anyone got rid of this ingenious system God only knows.
Anyway, Britain’s decree to adopt the Gregorian calendar on March 25, 1752, changed two things:
First, 11 days had to be axed from the calendar.** I don’t know who had a grudge against September, but that month was chosen to do the deed. September 2, 1752 in Great Britain and her colonies was followed by September 14, 1752.
Did this mean if your birthday fell on September 3rd you didn’t get a year older? No, most folks just pushed their birthday 11 days.

From the time of the switch onward, Washington chose to observe his birthday on February 22 rather than February 11; February 22 was celebrated as Washington’s birthday by Americans even when he was alive.
Second, 1752 was the shortest year ever in the British calendar. At just 9 months, it began on March 25 and ended December 31.
Thus Washington’s 20th birthday on February 22, 1753 was exactly 365 days after his 19th birthday on February 11, 1751.
If you’re not confused yet, go here.
*[For millennia the beginning of Spring (what we'd consider February or March in the Northern Hemisphere) was considered the beginning of the year. The Roman calendar we use today was an exception rather than the norm. And in fact, even Romans considered March the first month of the new year up until 153 BC. (See Why January 1st Marks New Year) It took centuries for that change to take hold in rural agrarian society.]
*[Astronomers under Julius Caesar suggested that inserting an extra day (leap day) in the calendar once every four years would make up for the fact that the solar year was actually 365.25 days, not 365.
It took fifteen centuries to determine that even those calculations were slightly off, by just under 1 day per century. So Pope Gregory instituted a new calendar, removing one leap day every century. But to make up for lost time---or gained time depending on how you look at it---societies adopting the Gregorian calendar had to skip 11 to 13 days on their calendars.]
Links:
www.crowl.org/Lawrence/time/britgreg.html
www.mountvernon.org/education/biography/born.html
George Washington: Unanimous my Animous


