You can find out the dates of most major religious and government holidays from The World Almanac or another good book of miscellaneous facts. You can look up more information about holidays in most general dictionaries and encyclopedias.
The names and dates of U.S. government holidays are listed in Section 6103 of Title 5 of the United States Code, and national observances (e.g., Mother's Day) are listed in section 141 of Title 36. State holidays are likewise listed in the respective state statutes.
In addition, U.S. government holidays back to 1997 are posted by the Office of Personnel Management.
Jewish holidays from the current and following year are posted by the Orthodox Union.
Worldwide holiday calendars are posted by Country Reports and Time and Date.