Which of the following is NOT a national U.S. holiday?

Valentine's Day is not a national U.S. holiday, which makes it the correct answer to this question. While it is widely celebrated and holds significant cultural importance, it does not have the official status of a national holiday.

In contrast, New Year's Day, Thanksgiving, and Christmas are all established as national holidays in the United States. New Year's Day marks the beginning of the year and is observed on January 1st. Thanksgiving occurs on the fourth Thursday of November and is a time for family gatherings and giving thanks. Christmas, celebrated on December 25th, is both a religious and a cultural holiday observed by many Americans.

Each of these holidays has been recognized at the federal level, leading to time off for federal employees and specific observances in various states and communities. Therefore, recognizing the distinction between widely acknowledged cultural celebrations and those that hold the designation of national holidays is essential in understanding U.S. holidays.