First off, I don't cater or cook for hire, but I'd want to get some things addressed before I signed on.
Since it's for charity, and they're donating food, I'd be concerned that they're going to ask you to be really tight on your fee, "since it's for charity". For a couple of cooks, sure, I can see a reduced fee or even donating your services.
But for every weekend for 6-9 months? That's a hell of a commitment. What other catering work or comps will you miss out on because of your obligation to them? What if you can't do a weekend because of some sort of personal circumstances? Do you have to get someone to cover for you? Is food all that they're donating? What about fuel? Service-ware?
How often are you to be paid? After every cook? Or at the end of the contract? Does the local store cut you a check or do you have to wait for it to come from corporate?
There's lot of questions and variables that you need to have spelled out and both parties absolutely clear on. Just protect yourself and remember that sometimes the most profitable job is the one you walk away from.