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

Midsummer Day is not a recognized national U.S. holiday, making it the correct answer to the question. The United States officially celebrates several national holidays, which include Memorial Day, Independence Day, and Thanksgiving.

Memorial Day honors those who have died in military service to the country and is observed on the last Monday of May. Independence Day, celebrated on July 4th, marks the adoption of the Declaration of Independence in 1776, a pivotal moment in American history. Thanksgiving, held on the fourth Thursday of November, is a time for giving thanks and commemorating the harvest.

In contrast, Midsummer Day, which tends to be celebrated in various cultures and is often associated with the summer solstice, does not have any formal recognition or celebration at the national level in the United States. Therefore, it stands apart from the other three choices, all of which are established holidays in the U.S. calendar.