It all started with a blog post by Neil Gaiman:
I propose that, on Hallowe'en or during the week of Hallowe'en, we give each other scary books. Give children scary books they'll like and can handle. Give adults scary books they'll enjoy.
Then it gained a name and momentum on Twitter: All Hallows Read. The new "tradition" is getting media coverage, and now it even has a website.
I like this new tradition. I think I may indulge in it.