Any time the ground is not frozen is a good time to plant trees. Provide extra water in the wintertime, and extra mulch, though. As always, mulch at the bottom of the hole, in the middle (half way up) and again on top of the surface. Then cover the top mulch with a layer of soil. Mulches include old leaves, hay, straw, rocks about the size of your fist, and in some places seashells or other soil modifiers. Old leaves are best!
A handful or five of ashes in the bottom of the hole dresses the bottom mulch. A dressing of stale urine (aged more than 6 months until it ferments and becomes less acidic) or composted manures is a helpful treat to the middle mulch layer, but make sure it is well aged or you'll burn the plant. But dressings are not necessary.
Trees require tillage like any other crop, and should be planted in east/west rows in Colorado to shade the hot summer sun and break the cold north wind.
Planting dates aside, it is never too early to dig the tree hole: dig it now, fill it with leaves or straw and let it compost for a little while. It'll ripen just in time for the trees! Then stir it by pulling it out to plant the tree and backfill with a little bit of ashes or your favorite dressings. You may like the smell of lilacs, but you'll be more excited about the fungi, viruses, retroviruses and bacterias you'll smell this winter, I promise!