Tree Planting
Early spring and late Autumn are equally good for planting trees, the key is that you want the tree to be as dormant as possible (ideally pre-budding or post-leaf) as this minimizes the risk to transplant shock.
Summer is poor both because its the middle growing season, but also due to higher risk of drought. The stress the tree is under makes it more likely to die.
Its not impossible to plant is summer, you just lose more trees that way, and you need to care for them more aggressively.
Winter would work mostly fine, except in our climate the ground freezes, which makes digging impractical and expensive.
Also you can't let exposed roots freeze, that will kill the tree, though if your snappy about planting it.....it should still be fine.
Although I'm sure there are exceptions, the only tree I know of with a clear seasonal planting preference is OAK, which does much better w/spring planting for some reason.
They won't be planting Oaks, so no worries, I expect you will see trees this fall.
