The best time to plant a tree is in the late winter or early spring, according to About.com. The next best time is in the fall. Summer is the worst time because plants can become vulnerable to heat damage.
Some people choose to plant a tree in the early winter, so they won't have to fight the icy soil later on. While it is possible for newly planted trees to survive the entire winter, the freezing temperatures can hinder the watering process, which is vital for new plants. The end of winter, when the ground is likely thawed out, is more suitable.