Is Christmas a proper noun or common noun?

The Word 'Christmas' is a proper noun, the name of a specific holiday.

A proper noun is the name of a specific person, place, or thing; 'Christmas' is a Word for a specific holiday (a holy day for some), the name of a thing. Proper nouns are always capitalized.

Examples of common nouns (synonyms) for the proper noun 'Christmas' are holiday, holy day, celebration, etc.