Although this holiday started out as a celebration of Washington’s birthday, as you can see from the excerpt from Wikipedia below, a very keen marketer made it a bigger holiday.
As the first federal holiday to honor an American citizen, Presidents Day was celebrated on Washington’s actual birthday, February 22. 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” in some sense a misnomer, since it never lands on Washington’s actual birthday, February 22.
An early draft of the Uniform Monday Holiday Act would have renamed the holiday to “Presidents’ Day” to honor the birthdays of both Washington and Lincoln, which would explain why the chosen date falls between the two, but this proposal failed in committee and the bill as voted on and signed into law on 28 June 1968, kept the name Washington’s Birthday.
By the mid-1980s, with a push from advertisers, the term “Presidents’ Day” began its public appearance. Although Lincoln’s birthday, February 12, was never a federal holiday, approximately a dozen state governments have officially renamed their Washington’s Birthday observances as “Presidents’ Day”, “Washington and Lincoln Day”, or other such designations. However, “Presidents’ Day” is not always an all-inclusive term.