Orientation day.
Seems a little far-fetched, but not impossible. Usually, in military handling of large personnel movements, they like to group people together for paperwork and orientation days. "This is what's about to happen, these are your points of contact, don't walk on the grass…" That sort of stuff.
I am not sure why they would go along with such a peaceful march to the gallows, though. So I suspect we aren't quite to that stage just yet. They are probably convening for something on their terms/plan, thinking it will help.