Suitable planting times depend a lot on where one is located. Generally,
planting of larger plants (trees, good sized shrubs) should be done when
the plant is not in active growth and temperatures are cool - fall
through early spring, if your ground is workable and weather permits.
This reduces the likelihood of transplant shock and the need for constant
watering. That doesn't mean they can't be planted at other times - just
that additional care will have to be taken. Even in my climate of very
mild summer temperatures, it is not recommended that planting be
undertaken in summer. May be best to get them in the ground ASAP and
watch the watering carefully going forward.

pam - gardengal
Ageed, However container grown trees are much more forgiving than bare root
plants which almost have to be dormant when planted. Ther are however
exceptions to every rule.