Graduation -- the time when 7th Year students graduate from Hogwarts -- will take place during this time. Promotion of other students to the next class year will also take place.
All posts to be counted for this term MUST have taken place by this time! Once your account has been either promoted or failed by the staff, it cannot be altered until the next term. Promotion can take anywhere from 2 to 6 hours, so you will need to be patient during this time!
In addition, once all promotions have taken place, all student accounts will be able to post in the London forums (like the Leaky Cauldron, Diagon Alley, etc). Keep watching the scrolling message bar at the top for when this will begin.