Who declared it President’s Day?

Today we celebrate Washington’s Birthday. Yet on every calendar I own, today is noted as President’s Day, where we supposedly honor both George Washington and Abraham Lincoln, the latter of whom’s birthday was the 12th. (Another notable President, Ronald Reagan, would have been 94 on the 6th.)
Per Matthew Spaulding, a Heritage Foundation scholar, in today’s Federalist Brief (No. 05-08):

“Although it was celebrated as early as 1778, and by the early 19th Century was second only to the Fourth of July as a patriotic holiday, Congress did not officially recognize Washington’s Birthday as a national holiday until 1870. The Monday Holiday Law in 1968–applied to executive branch departments and agencies by Richard Nixon’s Executive Order 11582 in 1971–moved the holiday from February 22 to the third Monday in February. Section 6103 of Title 5, United States Code, currently designates that legal federal holiday as ‘Washington’s Birthday.’ Contrary to popular opinion, no action by Congress or order by any President has changed ‘Washington’s Birthday’ to ‘President’s Day’.”
So how did this come to be known popularly as “President’s Day”?