All humans have faded except buffy. Dawn lasted til christmas and then spike and buffy are reunited

Hey, I'm trying to find a fic where Buffy believes she is the last human alive because all others have disappeared. I think at one point, Spike says that humanity's time is just over. Dawn lasted until christmas I think and Buffy was with her when she faded. After, Spike and Buffy find each other but she starts to fade too. I think it ends with the scoobies in the high school library. Any help finding this fic would be much appreciated! Thanks.

I am sure this is Every December Sky by Doyle on AO3. It's an absolutely fantastic fic!

https://www.archiveofourown.org/works/12511712