Presidents' Day is observed every year in the United States on the third Monday of February.
As a federal holiday, its official legal name remains "Washington's Birthday." George Washington's actual birthday is February 22 (or February 11, if going by the Julian calendar that was in use in the colonies at the time of his birth). It was established as a federal holiday in 1885. Then, by the Uniform Monday Holiday Act of 1968, which was designed to create more three-day weekends, observance of the holiday was moved to the third Monday of February (which never actually coincides with either of Washington's birthdays).
Presidents' Day later arose as an idea to honor all past presidents of the United States (but probably especially Abraham Lincoln, who also happened to be born in February), and it has since become a commonly used name for the the observance of Washington's birthday. In many states, including California, where Lincoln's birthday was formerly a legal holiday, the state holiday on the third Monday of February is referred to even in official channels as "Presidents' Day" (although, interestingly, the California Government Code does not seem to name the holiday at all). Most likely, this was done in order to sadden public school students by consolidating what used to be two days off into a single joint holiday.