why,if i don’t start my 2002 buick rendezvous within 7 days does my battery go dead

You might need a new battery.

you probably have a “draw” on the battery. in simple terms that means that something is “stuck” on. you will need to have a draw test done to determine if it is a draw. it could be anything electrical system including the alternator

The battery is running down. It could be the battery. How old is your battery? It also could be a dark load. That is something using power when the car is parked and it appears nothing is using power, but something is. Often antitheft or other features use power when apparently is doing nothing.

It is also possible that something is using more power than it should. Only testing can tell.

I’m with Jim gray on this one. you have a draw on your battery of more than 50 milliamps. Need to do a draw test to determine. The modules on these are designed to “go to sleep” after a certain period of time. It sounds like you have a module “staying awake”.