When was the Day of Severance?
I believe the Day of Severance is the day Ginko is sent out into the battlefield . Before this, tensions were rising at each side of the Wall - The young bears were brainwashed into thinking fighting in this battle would gain Kumalia's love, And you can see the human's disdain for bears when Kureha is picked on as a child for being a 'bear-lover'.

The Wall of Severance was erected after the battle (hence the Day of Severance) to separate the two worlds. 

There seems to be an oversight in the plot in regards to the Door of Friendship and when it was built, which is what I personally found confusing. It appears that the door of friendship was built prior to the day of severance, but only discovered in the midst of the battle. It does seem a little odd how this portal was overlooked for so long if it already existed, and what use it served without the wall in place.

Neither world seems to know much about the portal, it seems to be a only known about by Ginko, Kureha, Reia and Lulu. One theory is it could have been built by Kumalia, another is that it's a plothole.