Originally established to celebrate Washington and Lincoln, its been expanded to be a somewhat generic observance of Presidents. Its tricky figuring out when Washington was really born because it was prior to the adoption of the Georgian calendar, so its was February 11th, which now falls on February 22nd. Lincoln was born on February 12 (on the same day - the actual day and year - Charles Darwin was born).
Since 1971 these birthday's have been celebrated on the third Monday of February following the passage of the Uniform Monday Holiday Act.
- Background from about.com and wikipedia.
Constitutional historian Akhil Reed Amar offers some general comments about the Presidency.
