Mothers Day is a popular tree planting event, however I've sacrificed more trees to the weather patterns around mothers day than any other time.
The best time to plant a tree is when it is dormant. Spring is not the best time if the weather had already warmed up and the plant is in bud or has leafed out.
If you must plant in the spring; watch the weather patterns closely and try to time it in a cool down. Make sure the tree does not have too much water or too little, using a simple potted plant water probe to check wetness.
Tree health is about reducing the stresses on the tree. Replant shock is maximizing the amount of shock the plant can take so any OTHER shock, such as a week without water, insect attack, or too much nitrogen from the lawn fertilizing, can tip the balance and start the tree in what is called a death spiral.
The best time to plant is in the fall or even winter if the ground is not frozen. A decidua tree is hibernating and will not even notice the change. The problem with this plan is it is often freezing and you can't get any water on the tree. You'd think that because the tree is dormant it doesn't need any water. But we don't want those freshly cut roots to desiccate all winter long. There are usually little warm ups which will trigger small shoots of roots and if that ground is dead dry, those roots will die.
The answer is to use a watering device, I'll get one on the blog page here for new plantings, that basically is a slow release of water. If it is not too full it can freeze and not rip. But if there is a warm up it will automatically wet the tree as it needs it. This same device is good for any newly planted trees.
Over watering does as much damage as under watering. So I like to plant by digging drainage trenches out from the planting hole back filled with organic matter and drainage rock. This does two things; first it helps keep us from overwatering. We can water the tree pretty liberally because we know excess will drain off and not drown the roots. The second thing it does is give roots an avenue they will want to go out that will give the tree a faster stable root mass.
See the Tree section above on the airspade for what those trenches look like.