The best time to plant your tree is as soon as possible upon receiving it.
Each tree comes with detailed care and planting instructions. If you are unable to plant the tree due to weather conditions, remove the plastic shipping bag from around the container, keep it well-watered, and it will be fine in the pot for several months.
Trees can be planted year-round, with the best times being in the fall (September - November) and spring (March - June). During the winter months, trees are dormant (see winter FAQ), but the ground is often frozen which makes digging difficult.
When planting during the summer months, be sure new plantings are kept well-watered.