The 3rd Monday of February in not officially titled "Presidents Day". It is officially titled "Washingtons Birthday", in honor of George Washington which was originally implemented by an Act of Congress in 1880 for government offices in the District of Columbia (20 Stat. 277) and expanded in 1885 to include all federal offices (23 Stat. 516).
As the first federal holiday to honor an American citizen, the holiday was celebrated on Washington's actual birthday (February 22), and on January 1, 1971, the federal holiday was shifted to the third Monday in February by the Uniform Monday Holiday Act.
This date places it between February 15 and 21, which makes the name "Washington's Birthday" a misnomer, since it never lands on Washington's actual birthday.
Reference: http://en.wikipedia.org/wiki/Presidents%27_Day
