It's been a few yrs for my son, but he got his assignment, and roommate, around midsummer, like July. They contacted each other to see who was bringing what.
Yes parents are there for move-in. Probably most, unless geography prohibits (lots of internationals at CMU). I remember a line on the orientation schedule: around 5:00 move-in day, "Parents say goodbye". That was a good idea, to get rid of all us sobbing embarrassments at the same time so the kids could have some fun. The orientation is a good one...long, well organized and lots of fun (what he remembers

).
He was in Donner, second choice, deposited around April 5. Ugly on the outside but big rooms, convenient location, very social. He liked it.