How do I prepare the soil, from a hole that I have already dug, for the proper nutrients to accept a bare root dwarf nectarine tree? I have not yet purchased it, should I purchase it from a local nursery only where it has been acclimated to the climate zone?
What month do I plant it?
I live in the Foothills of the Sierras in Northern California. I don't know the climate zone number for this area.

Howdy there, yuba person , from valley man of California.
Plant , when they come in at the stores. Home Depot guarantees everything to grow, so save your receipt. They'll be in , next month or two.
If you have good soil, just backfill your hole with, around equal parts your soil to compost or mulch. With just a bit of manure mixed in.
Then, side dress with manure.
Then water in.
Make furrow at drip line. That's where most of feeder roots are.
If you want a tree that will stay small, like say, 4 to 6 feet tall. You need to get what is called a ( True Dwarf ) The others ! There is nothing dwarf about them. After several years, they resort back to normal, standard trees.
Enjoy your Holiday's 😀
😎😎😎

Don't add anything to your planting hole unless your native soil is just really horrible. Trees root and expand out of the planting hole much better and more quickly if their only choice is native soil.

The best time to plant is January when trees are at their most dormant and the nurseries have the most choices. Home Depot doesn't necessarily stock trees that will do best in your area. The Yuba City/Marysville area is big enough to have a couple local nurseries - go look at them. Your best bet, brand wise, is Zaiger or Dave Wilson grown trees, all grown in the Central Valley on appropriate root stock.

In the spring, after the tree leafs out, fertilize lightly. Late spring and mid summer, fertilize again.

The life expectancy of a standard sized nectarine is 12 - 15 years. For a Dwarf, its 5 or 6 years.

As Daisy said, it is no longer recommended to amend native backfill soil unless absolutely unavoidable. Amending can cause problems with water movement through the soil because of the interface between the different materials, plus organic amendments continue to decompose over time causing the tree to sink.