User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-7153535-20130710153323/@comment-3146930-20130712220903

The Lost Hero had Ma Gasket and her sons reform, the Earthborn at the Wolf House, and most of the other enemies were gods or undead people, meaning the Doors of Death had been open by December. Also, Nico managed to sneak Hazel out of the Underworld because Thanatos had been captured. Hazel knew Jason before he vanished in October, so Thanatos must have been captured before then.