Why Everyone? Your question embraces false equality. America is and always was a Christian nation. The religion that guided our country's founding is not equal to all other religions.
What Year Is It? We here in America mark time as a way to honor our deliverer. Anyone can say Jesus is not my deliverer. That is irrelevant. His birth is honored at the Founding and we keep honoring him every day. In the Year of Our Lord, February 1, 2021. Other religions may record differently but this is how Christian nations, like ours, mark time.
Why Important? It's the same basis that I know that air is important to everyone in the country! I believe in the 1776 Commission. That is, I believe it is important for everyone in this country - and the world - to keep in mind the Founding principles of America, aka the Spirit of '76.
The Left hates the idea of American exceptionalism and even distort what it means. Simply put, the exception is that America's government was founded on the principle of limited government. America is the only country in the history of the world based/founded on individual rights and limited government. This idea can be and should be frequently traced back to its source.
Render to Caesar the things that are Caesar’s, and to God the things that are God’s.
Jesus, Luke 20:25