Usually best time is very very early spring.
You want them to be in the ground and growing enough to establish a good root system before winter.
That's what we do in Ala. zone 8a.

any time except the heat of summer can be done. The heat will stress them. But no matter when you plant them, make sure they are watered often. I planted trees in mid-summer in Houston and they survived, but there was a lot of watering. When I planted them, I left a little trench all around the edge of the hole and filled that with sand instead of dirt of that water would get down around the root ball. So if you plant at this time, make sure that you are able to get water to the roots.

Ask either your local Master Gardeners or the staff at a good nursery. Either should be able to give you advice about how specific trees can handle later planting.

(if you don't know how to find Master Gardeners, google "Master Gardeners" + your county and state names.)

in zone 6. Growing like a green bay tree. A year and a half later it has apricots starting.

My understanding is they grow roots over winter so they can stand the stress of summer heat better.