Answer: Washington’s Birthday.
Washington’s Birthday was first declared a federal holiday by an 1879 act of Congress.
The Monday Holiday Law, enacted in 1968, shifted the date of the commemoration of Washington’s Birthday from February 22 to the third Monday in February, but neither that law nor any subsequent law changed the name of the holiday from Washington’s Birthday to Presidents Day.
In another oddity, while George Washington’s birthday was/is February 22, that isn’t actually the day on which he was born. Washington was actually born on February 11, 1732 based on the Julian calendar that was then in use in the British Colonies. However, when the Gregorian calendar was adopted in the English Colonies (1752), he opted to begin observing his birthdate anniversary on the equivalent date; February 22.
When the federal holiday was shifted to the third Monday in February by the Uniform Monday Holiday Act, that moved “Washington’s Birthday” to sometime between February 15 and 21 – which, of course means that “Washington’s Birthday” never actually occurs on…Washington’s actual birthday.