We know that the Day of Severance had to have happened some time after Kureha was born. Indeed, it must have occurred sometime after Ginko and Kureha became friends (i.e. after they met on the snowy battlefield past the Door to Friendship), since they could not have become friends in the first place had Severance already occurred.
We also know that the Day of Severance happened before episode 1 of the show, because, of course, that's how the narrative begins in the first place. I'm pretty sure that Severance was also before Ginko and Lulu met.
Can we pinpoint when the Day of Severance happened any more accurately (relative to other events in the story)? In particular, it's not clear to me whether the Day of Severance was before or after the day on which Reia sent Ginko back across the Door to Friendship.
(Or maybe I've got this all wrong? In episode 11, the way Kureha's classmates harass her about her friendship with bear-Ginko [during a flashback] seems to suggest that the Day of Severance had already happened - so does that mean that the Day of Severance even precedes the day on which Kureha went across the Door to Friendship to fetch Kureha from the snowy battlefield?)