Why not Congress Day, Judiciary Day, or Separation of Powers Day?
Presidents' Day may be a good time to ask why we have a national holiday to celebrate the executive branch of government, but don't have a Congress Day or Judiciary Day. Presidents already get a disproportionate share of the credit and blame for anything that happens in government. The president also has a major advantage over Congress and the Supreme Court because he is almost the only government official whose name even generally ignorant voters know.
While Presidents' Day has its origins in celebrations of Washington's birthday, over time it has come be understood as honoring all the presidents—even those who clearly don't deserve any honors from posterity. Perhaps the time has come to redress the balance by giving the other two branches of government holidays of their own. Better still, we could have a Separation of Powers Day, or even (following Federalist 51), an Ambition Counteracting Ambition Day.