People celebrate Columbus Day primarily to commemorate Christopher Columbus's landing in the Americas on October 12, 1492. The holiday marks the beginning of recorded history in America from a European perspective and recognizes Columbus as a significant Renaissance explorer who founded the first permanent European settlement in the New World. It also celebrates the contributions of Italian Americans and their heritage, as Columbus was an Italian explorer from Genoa, and the day became a symbol of pride for Italian immigrants who faced discrimination in the U.S. Columbus Day has been a federal holiday in the United States since 1937 and includes celebrations of cultural exchange between America and Europe initiated by Columbus's voyages.

Additionally, Columbus Day has historical significance because it was established partly in response to events like the lynching of Italian Americans in the late 19th century, aiming to honor Italian American contributions and promote patriotism. It is also a day on which themes such as citizenship, loyalty to the nation, and social progress are emphasized.

However, it should be noted that the holiday is controversial. Many people and governments now also observe Indigenous Peoples' Day to recognize the Native populations whose lives and cultures were deeply disrupted by European colonization initiated by Columbus's voyages.

In summary, Columbus Day is celebrated mainly to honor Columbus's historic 1492 voyage, his role in European colonization, and Italian American heritage, alongside a broader celebration of national pride and cultural exchange.