I too am sorry for your loss, and well understand your need to make sure it thrives. How tall is the tree and what size pot is it in now, also can you see any roots coming out of the bottom of the pot? I would harden off in a sheltered area away from any potential cold winds and don't over-water. Dig your hole quite a bit bigger than the pot size as suggested, but make sure you don't create a "sump" where the water will just sit there, especially if you are on heavy soil (test the hole first by adding a watering can of water and see how it drains before planting). If it doesn't drain very well, add some sharp sand / grit and fork into the bottom and sides. Again, depending on the nutrient condition of your soil, perhaps a light sprinkling of bonemeal added to the excavated soil may help it establish? Remove your tree from its pot by holding onto the main stem and gently knock the top edge of the pot with your hand, then put the tree into the hole and turn it around to get the best view as to how it will eventually grow. I doubt any roots will have grown enough to either coil round the bottom of the rootball or make a fibrous mat but, if they have, gently tease some of them out with your fingers. Before back-filling the hole, I would knock in a tree stake right against the rootball. Back-fill some of the hole with the excavated soil and gently firm the rootball with your foot and just add a little bit of water to help with the soil settlement. Keep doing this until you have used up all the soil. Get a rubber tree tie and secure the tree to the stake ensuring it is not too tight because you don't want to harm the trunk as it grows until it no longer needs supporting. How about getting a nice brass plaque inscribed which you can insert into the ground by the tree - not that you would need a reminder, but it's just a thought. Memories are made of things like this, and I wish you every success.