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

Halloween is not a national U.S. holiday, which is why it is the correct answer. While Halloween is widely celebrated on October 31st with various festivities such as trick-or-treating and costume parties, it does not hold the status of a federal holiday. In contrast, Columbus Day, Martin Luther King Jr. Day, and Veterans Day are all recognized as federal holidays, meaning they are established by law and observed at the national level, providing federal employees a day off and often recognized by state and local governments as well. This distinction highlights the nature of federal holidays as official observances that carry specific significance in the United States.