Like Arron said, no worries about digging the holes ahead.
Do expect to make adjustments to those holes once you get the trees on site to plant, holes should be twice as large in diameter as the
root ball but don't dig deeper than the depth of the root ball or fill in and tamp gently, the trunk needs to be at ground level when planted, not deeper and not higher.
You can use amended soil to do the filling and water in as you fill to help the soil settle in around the
roots faster.
For evergreens (conifers) you can plant them at the same time you plant deciduous trees (dormant stage), this allows the trees to wake up in their new home soil.
Once you have them planted and watered in, just watch that soil and don't let it dry out to bone dry.
In the spring as the buds start to open and the new growth begins on the conifers, water them all with a diluted vitamin B-12 solution to get the roots growing faster.
Redhawk
(I always like to plant trees at any time between December and February since we now start to see no frost around mid March).