Christmas Day did not become a federal holiday until 1870 when President Ulysses S. Grant declared it a legal holiday. Rep. Burton Chauncey Cook (Ill.) introduced HR 2241 in the 41st Congress to make the day a holiday in the District of Columbia. After the Senate and the House agreed on the final wording, President Grant signed it into law on June 28, 1870. The act (a facsimile of which is available at the Library of Congress Century of Lawmaking for a New Nation website here) reads:
An Act making the first Day of January, the twenty-fifth Day of December, the fourth Day of July, and Thanksgiving Day, Holidays, within the District of Columbia. Be it enacted by the Senate and House of Representatives of the United States of America in Congress assembled, That the following days, to wit: The first day of January, commonly called New Year's day, the fourth day of July, the twenty-fifth day of December, commonly called Christmas Day, and any day appointed or recommended by the President of the United States as a day of public fast or thanksgiving, shall be holidays within the District of Columbia, and shall, for all purposes of presenting for payment or acceptance of the maturity and protest, and giving notice of the dishonor of bills of exchange, bank checks and promissory notes or other negotiable or commercial paper, be treated and considered as is the first day of the week, commonly called Sunday, and all notes, drafts, checks, or other commercial or negotiable paper falling due or maturing on either of said holidays shall be deemed as having matured on the day previous. APPROVED, June 28, 1870 by President Ulysses S. Grant