The plural form of "Christmas" is "Christmases." The plural of a proper noun, which is the name of specific person, place or thing, is often formed in the same way as the plural of a more general, or common, noun.
In general, most proper nouns are made plural by adding the letter "s." Therefore, all members of the Smith family are referred to as the "Smiths." However, if the name ends in a "z" or "s" like "Christmas" does, the letters "es" must be added. If a proper noun ends in the letter "y," an "s" is placed at the end. The days of the week, such as "Sundays" or "Fridays" are good examples of this rule.