Which of the following is a national U.S. holiday?

New Year’s Day is recognized as a national holiday in the United States and is officially observed on January 1st each year. This day marks the beginning of the new year in the Gregorian calendar, which is the calendar used by most of the world.

New Year’s Day is celebrated with various traditions including fireworks, parties, and resolutions for the coming year. Government offices, schools, and many businesses are closed on this day, which signifies its status as a national holiday.

In contrast, the other holidays mentioned do not carry national recognition. Valentine's Day, celebrated on February 14th, is primarily a cultural and commercial holiday dedicated to love and affection. Mardi Gras is a festival celebrated with parades and festivities, particularly in New Orleans, but is not a national holiday. Earth Day, observed on April 22nd, promotes environmental awareness and action but, like Mardi Gras and Valentine’s Day, does not have national holiday status in the U.S. Thus, New Year’s Day stands out as the correct answer due to its official recognition and observed celebration across the nation.