Reading Dawnseeker's quote regarding the patch notes, it'll be fair to say that to be able claim Day 5s award you need to have defeated The Horseman in the Heroic Realm, not the main quest.
EDIT: I think Tynon are aware there is a problem with Day 5 quest completion. There gave been a lot of players beating The Horseman before the quest expiry and NOT being able to claim their rewards.
