What if you’ve had lots of rain & humidity. Our area is slightly sloped. The ground is clay, rocks & some sand. I’m concerned about having concrete poured on a wet ground. We need to have the area graded and sloped slightly in the opposite direction, away from the house. If it’s too wet, will the new patio be unstable? I’m worried about cracking & flaking if the ground isn’t dry enough.
Civil engineer here. The concrete needs to stay wet in order to cure properly. As noted before, concrete doesn't dry out to harden, it cures through a chemical reaction that needs water to facilitate the reaction. If the ground is dry, the ground will absorb moisture from the concrete and it won't cure properly. The ground should be very moist and compacted as well as you can manage.
Also, wet clay is notoriously weak and you may end up with uneven support under the concrete, which will then lead to it cracking and settling after it's cured. Hard to say without seeing it but if you can remove as much clay from beneath the concrete slab, you'll have less chance of it settling and cracking. This is especially true if the clay is expansive. It will expand when wet and shrink when dry and your concrete slab will be a disaster. Best to get as much clay removed as possible, compact the sub-grade evenly, and make sure to water the ground before placing the concrete.
Have you thought about proper joints? Concrete will crack. If you can get it to crack along a joint, no one will see it. Look at some professionally done concrete flatwork and you'll see what I mean. Good luck.
Damp ground will not be a problem unless is can not support the weight of the concrete. Cement/concrete does not "dry" , it cures , a chemical reaction. It absorbs water to cure, as noted , some concrete applications are sprayed with water to aid the cure.
I would wait until it dries up a bit. The concrete has a lot of moisture that needs to go somewhere. If the ground is too wet (clay doesn't absorb water) and the air is humid, you are probably asking for trouble. I see your problem all the runoff heads toward your house at worst or toward the lower right side at best. I suggest waiting till it's pretty dry. We had a similar problem with building a shed in our backyard. The best place was low and rain kept that area really wet. We put an 8" foundation in when it finally dried up. That foundation did the job. We also built up the sides of the foundation with fresh soil. The wettest area became normal.