The date you're probably looking for is on the Season Calendar:
http://www.usfirst.org/roboticsprograms/frc/calendar/
Initial capacity usually leaves at least 10 slots, sometimes more if big area growth is anticipated, for rookies who haven't registered yet.
If the rookies don't show, then waitlisted teams can get them. Sometimes not enough slots are reserved for rookies and the Regional asks for veteran team to voluntarily transfer elsewhere.