Oh wow, this is an absolutely brilliant story, with so many perfect and hilarious details! Freddie Mercury's Christmas covers! Hastur and Ligur and the Horspersons fanboying each other! Adam nearly destroying Christmas because he misses the Them (and of course, they're the ones to fix it, not the supernatural beings)! And this wonderful dialogue:

“There’s some sort of disturbance.” Aziraphale tried to pick up on it, sensing the air around him. “As though something bad is coming.”
“On Christmas?” Crowley said.
“That’s not a very demonic thing to say.”
“What if I told you I was hoping your answer would be ‘yes’?”
“I wouldn’t believe you.”

Awwwwww, just awwwww. <3