I am often asked when to plant trees purchased from the nursery. The question has both a long and a short answer, and I’ll begin with the short version. Note that this article specifically addresses when to plant trees in areas with cold winters; if you live in a warm climate, then essentially any time of year except summer is a good time for planting (the summer heat presents too great a threat to young trees).
In general, the best time for planting trees is late winter or early spring. If that doesn’t fit your schedule, then aim for autumn. Summer is a bad choice because the weather is too hot and the actively growing plants are too susceptible to damage. Weather also restricts your options in the winter (at least in the North), because the cold causes the ground to freeze. If you’ve had the foresight to do all your digging ahead of time (before the ground freezes), it’s not impossible to plant trees in the Winter. But unless you can water them sufficiently, early-to-mid winter is not the best planting time either.