In some languages, “Christmas” is a plural noun, and since the English word for Christmas ends with an “s”, it sometimes makes English learners think they should use a plural verb with it.
However, the English word Christmas comes from Christ’s mass, and “mass” is a singular noun. There is no reason to say “Christmas are”; the only correct form is “Christmas is”:
This naturally applies to all verbs, not just “to be”:
Also, don’t forget that the corresponding determiner is “this”, not “these”:
and similarly for “that”—we would say “that Christmas”, not “those Christmas”.