Christmas is a proper noun. Proper nouns are used to name specific people, places, or things and are always capitalized. Since Christmas refers to a specific holiday celebrated on December 25th, it is considered a proper noun.
For example, in the sentence “We celebrate Christmas every year,” the word “Christmas” is capitalized because it is a proper noun. In contrast, common nouns are general names for people, places, or things and are not capitalized unless they begin a sentence.