Always observe the "ten commandments" after seedlings are delivered.
Although you do not plant most wildland trees during the summer you should make sure you have ordered your trees for the season by early summer. Many people who wait until fall to find available trees just might not find any seedlings. Always order your seedlings as early as you can.
Planting urban trees is a little different. Horticultural planting has evolved into an all year operation because of the extra protection of a "root ball" with each tree. Any season is OK for planting balled or burlaped trees.