Dec, Jan, Feb
If you're buying mail order, open the package straight away. The tree pictured is a bare-root and doesn't come in a pot.
Plant the tree immediately. If you can't, heel the roots into damp soil, firm well and water to prevent them drying out.
Dig a deep, wide planting hole and check that it's large enough to accommodate the entire root system.
Add compost to the base of the hole and fork it in, and also mix compost with the soil you removed to improve it.
Hammer a stake firmly into the hole and position the tree. Saw off the top of the stake just below the bottom branches.
Hold the tree so that the roots are well inside the hole and fill in around them with the improved soil.
Press soil down firmly around the roots as you fill, to remove air pockets and give good soil contact with the roots.
Use a plastic tree tie to secure the trunk to the stake. Ties come with a rubber buffer to stop the stake rubbing the trunk.
Water well, then mulch with compost to keep it moist but, to avoid rotting. Don't pile it against the base of the tree.