The longer it sits in the fridge, the better off you are, up to the point where it starts to rot, but after a couple of days of rest, it's not going to make that much of a difference, especially if it's a young Cornish X.
If you do it tomorrow it should be fine by Sunday. I never had any problems with my birds being tough, stringy or anything else whether they were fresh or frozen.
I highly recommend a brine for any bird. My favorite is 1lb of salt, 1lb of honey and 1 gallon of water. Mix to dissolve salt and honey, add bird and let soak up to over night. The longer it goes the saltier it gets, so if you do not like salty foods maybe four hours would be a good place to start. However if you like it salty like I do I would let it go over night, or 8 hours. This brine makes a lot so you can reduce it to suit the number or size of the bird(s), just as long as the ratio remains the same. Good luck and good eats.
The bird i'll be processing is 5 1/2 weeks old. It is the biggest of the bunch and started to limp today. I don't know if it hurt itself or if its legs are going bad because of its wieght. It probally at about 4 1/2 lbs live wieght.