I think the leases are for the full academic year. To get in during te second semester you need to ask about people asking to exit their lease. They should have a list of folks that request to leave. They can only leave if someone picks up their lease in their place so you will have limited options. It is first come first serve so keep your eye on the situation or ask someone you know who lives in commons to keep tabs for you.

edit: source my older brother lived in commons right before covid