This day originally was observed in celebration of the Birthday of George Washington, the first President of the United States.
But, as time passed and more and more Presidents were elected, the name of this day was changed to "President's Day".
Under the new name of President's day, it is celebrated on the third Monday of February each year.
To read more about Washington’s Birthday and how it came to be, as chronicled by the National Archives, HERE’S THE LINK.
And if you would like to read Wikipedia’s ‘take’ on President's day, HERE’S THE LINK