You can begin planting apple trees in the early spring or late fall in much of the U.S. except the northern-most latitudes.
In that part of the country within USDA Zones 6 to 8, you can begin planting apple trees either in the early spring (before they leaf out) or in the late fall (after they've gone dormant). North of Zone 6, however, apple planting is pretty much limited to springtime . . . since the bitter winter weather in such areas would kill newly planted trees. (Most apples do poorly in the very warm Zones 9 and 10: There just isn't enough winter chilling to satisfy the plants' dormancy needs.)
There are only a few rules to be followed in planting, but they are important and should be observed closely. First, it's essential that the roots never dry out: Keep the tree out of the sun and wind, and make sure the roots are damp until they're safely underground. Next, dig a big hole . . . at least twice as wide and deep as the natural spread of the roots. (Of course, the graft between the rootstock and the scionwood must be above the ground: Otherwise the scion will send down its own roots, and the dwarfing action of the rootstock will be defeat ed.) Pack topsoil firmly around the roots, and trickle a bucket of water around the trunk after the hole is filled in. If you're planting a tree on Malling IX rootstock, it's going to need support, so drive a stake in now . . . and be sure to place a hardware cloth collar around the base of all trees to fend off hungry mice and rabbits.
Freshly planted apple trees should be pruned in order to compensate for the loss of feeder roots during transplanting. If you're growing one-year-old whips, simply cut the above-ground portion of the stem back about a third. Branched trees ought to be pruned to four or five strong branches . . . and then even these stems should be cut back to 3/4 their original length.
In future years, it's best to follow the recommendations of a good pruning manual: One that we've used is Pruning Simplified by Lewis Hill (Rodale Press, 1979).