Mid-August and early September is a great time to plant trees. Specifically- Pine, Spruce, and other Evergreens are good ones to plant so they can establish themselves before Winter comes around. Planting these trees too late in the season won't allow them to thrive before the onset of winter. Feel free to plant any tree during this month just to be sure it can establish itself before the cold weather comes, but deciduous trees and shrubs can be planted up until November.
Make sure to water your new tree everyday for three or four days and then gradually reduce the frequency of watering. When watering, slowly apply water to the rootball and surrounding soil. A thorough watering every seven to ten days, in dry weather, should be sufficient for three to four weeks after planting. Continue to water until the ground freezes.