Hi Scooby, and welcome to Club Arnage!!
A camp site ticket for the official areas allows you to camp form Wednesday 9th. You can arrive at the sites earlier (officially from Monday 7th) but you will need to pay a supplement to the person on the gate!
As far as I am aware, you have ot leave the campsites by the Monday after the race (14th) I've certainly not knbown anyone stay there Monday night before and the clean up operation is well underway by Monday afternoon so I would suggest you get a hotel (if you can find one) for the Monday night.
As JBP says, you need to have a permit for the camp istes and getting one form the ACO now will be pretty unlikely, you should be able to find something in the Sell/Swap section of this forum though.
Good Luck, Have a great time (I know you will!!) and call by Maison Blanche for a beer!!