Which of the following is NOT a national US holiday?

Valentine's Day is not recognized as a national holiday in the United States. National holidays, also known as federal holidays, are designated by the government and typically include days like Christmas, Independence Day, and Thanksgiving, which are observed nationwide and often entail closures of government offices and institutions.

Christmas, celebrated on December 25th, Independence Day on July 4th, and Thanksgiving, which falls on the fourth Thursday of November, are all established federal holidays where many businesses and schools close to allow for celebration and observance. Valentine's Day, on the other hand, celebrated on February 14th, is a cultural observance focused on love and affection between intimate partners, but it does not have the same official recognition or widespread public holiday status as the others mentioned.