Wait until at least mid septembe, maybe a little longer.
You don't need to ammend the soil, there's nothing wrong with clay. I wouldn't plant anything level with the ground per se but if you want to have a raised bed with other things in it, you will need to bring in soil and that's a pretty major project.

In your area, you could wait until full dormancy to plant. As QQ says, amending the soil is NOT a good thing! Just make sure you dig a hole that is three Xs wider than the root ball, just to loosen it up. Plant higher rather than level with the existing soil. IF area remains wet due to faulty drainage, then bringing in good top soil and constructing a berm might be an option. Be sure to mix the clay with the top soil rather than layering it on top. (We had to do that in the front yard.)

I have nothing but hard red clay in the back and all the trees have done fabulously. I did amend the large perennial beds prior to planting, but that will only be done once in my lifetime ;-)