When did congress first pass into law making Chrstmas a federal holiday?

Answer 1

The first five congressionally designated federal holidays were New Year’s Day, George Washington’s Birthday, Independence Day, Thanksgiving Day, and Christmas Day. Approved in the 1870s, they were applicable only to federal employees in the District of Columbia. In 1885, Congress began to extend holiday
coverage to federal employees outside Washington.

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,” and “in every State of the Union.”

Answer 2

Around 1870, when US Grant was president.