Here's why and how we can express it:
* Christmas is a relatively new holiday: The celebration of the birth of Jesus didn't become widespread until the 4th century, well after the decline of Latin as a spoken language.
* "Christmas" is derived from "Christ's Mass": The Latin phrase for "Christ's Mass" would be "Missa Christi". This is a literal translation, not a specific word for the holiday.
To express "Christmas" in Latin, you could use:
* "Dies Natalis Domini" - literally "Day of Birth of the Lord", a more formal and traditional phrase.
* "Nativitas Domini" - meaning "Birth of the Lord", a more accurate translation.
* "Festum Nativitatis Domini" - "Feast of the Birth of the Lord", emphasizing the celebratory aspect.
It's important to remember that Latin, while being a powerful and expressive language, doesn't always have a direct equivalent for every modern concept.