A plural noun is made possessive by an apostrophe.
You are confusing the final “s” in “Christmas” as a pluralizing “s” rather than its being part of the noun.
The correct way to pluralize a singular noun ending in “s” is to add an apostrophe and an “s.”
I call this “Jesus’s Rule.”
No, I’m not.
Maybe it’s my age, and style has changed, but I was taught in school to do it as I posted. (It stuck, perhaps, because my own name ends in ‘s’.)
https://www.thesaurus.com/e/grammar/whats-the-rule-for-doing-a-possessive-after-the-word-s/
https://grammarist.com/punctuation/apostrophes-with-names-ending-in-s/
(I’ve never heard prayer ended in church with ‘In Jesus-es name’.)