It was originally celebrated on George Washington's actual birthday, which is February 22. In 1968 the United States Congress passed the Monday Holidays Act, which changed the official observance of Washington's Birthday to the third Monday in February. The holiday can only occur between Feb. 15 and Feb. 21, which means that it is always after Lincoln's birthday and before Washington's birthday. Some political leaders wanted to change the official name of the holiday to President's Day to honor both Abraham Lincoln (whose birthday is February 12) and Washington but that proposal was rejected by Congress.
Liberals prefer the name "President's Day" because it converts the respect for George Washington, a devout Christian, into a secular worship of the office including liberals who have held it.
State governments are not bound to have a state observance of that day, but all fifty states and the District of Columbia do. 29 states and the District of Columbia officially call the day George Washington's Birthday. Georgia and Iowa are among those states, but they do not have a day off for state employees that day. 13 states officially call the day "President's Day". 5 states officially call the day "Washington's and Lincoln's Birthday". Alabama officially calls the day "Washington's and Jefferson's Birthday". In Arkansas, the day is officially "Washington's birthday and Daisy Gatson Bates Day". In Colorado, the day is officially "Washington-Lincoln" Day.
"According to the Gregorian or "New Style" calendar that is in use today, George Washington was born on February 22, 1732, but according to the Julian or "Old Style" calendar that was in effect in England and her colonies until 1752, his birth date was February 11th." 
Washington's Birthday was publicly celebrated in the late 18th Century when George Washington was still President of the United States. Washington's Birthday became an official federal holiday in 1885 when President Chester Arthur signed a bill making it a federal holiday.
President Lincoln's birthday, Feb. 12, never became a federal holiday but was celebrated as a legal holiday in many states outside the old Confederacy.