Why was Christmas declared a national holiday?

The first official US holidays were declared by Congress on June 28, 1870. New Years Day, Independence Day, Thanksgiving Day, and Christmas Day were all declared holidays in the first act. George Washington's Birthday was declared a holiday in 1871.

According to Title 5 of the US Code, neither Congress or the President has the authority to declare a holiday that must be enforced by the states - Federal holidays are only applicable to Federal employees and the District of Columbia.

Each state chooses which holidays it will recognize.

QUOTE:

"The act of June 28, 1870, which was apparently prompted by a memorial drafted

by local "bankers and business men," provided that New Year's Day, Independence

Day, Christmas Day, and "any day appointed or recommended by the President of the

United States as a day of public fasting or thanksgiving [were] to be holidays within the District." This legislation was drafted "to correspond with similar laws of States

around the District,"3 and "in every State of the Union."

senate.gov/reference/resources/pdf/Federal_Holidays.pdf