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

Valentine's Day is not recognized as a national U.S. holiday because it does not have the same legal status or federal observance as the other holidays listed. National holidays, also known as federal holidays, are designated by law and typically involve offices and institutions closing to observe the occasion.

Veterans Day, Memorial Day, and Christmas are all established national holidays. Veterans Day honors military veterans, Memorial Day commemorates those who have died while serving in the U.S. military, and Christmas is a widely celebrated holiday marking the birth of Jesus Christ. Each of these holidays has specific historical significance and are recognized across the nation with federal acknowledgment.

In contrast, Valentine's Day, celebrated on February 14, is primarily a cultural holiday focused on love and affection between intimate partners and does not have official federal recognition or mandated observance.