That is an urban myth that has been debunked:
Presidents Day
Claim: The federal holiday observed in the United States on the third Monday of February is officially designated as "Presidents' Day."
Status: False.
Bottom line: The official designation of the federal holiday observed on the third Monday of February is, and always has been, Washington's Birthday.
I always refer to the holiday as Washington's Birthday. I also refer to November 11 as Armistice Day.