If you can, put them in the ground as soon as possible. Hardy plants left in containers for the winter are much more vulnerable to damage or dieback than plants in the ground. Soil is insulating, and root hardiness is not as great as the cold hardiness of above-ground plant parts. The spruces can be planted any time the soil is workable (that is, not frozen and not soggy). While it would have been ideal to get them established before winter (I acknowledge your rationale for buying them now), they still might produce a bit of root growth during mild winter weather, which is preferable to them remaining stagnant in containers that constrain the roots.

If winter cold doesn't cause damage to plants over the winter, drying out definitely will, especially for potted plants that dry out much faster than in-ground plants. Any time the soil is not frozen (and therefore can be checked for moisture), water the plants as needed. The monitoring process is the same as it would be in summer: feel the soil a couple inches deep into the container (how deep depends on container size...at least an inch down) and water thoroughly when it becomes somewhat dry to the touch at that depth. If the soil has become quite dry and pulled away from the sides of the pot, it will be harder to rehydrate the root ball since at first the water will tend to run down the outside of the root ball and leave the pot too quickly to be well absorbed. Using tepid (not hot) water may help it be absorbed by cold (unfrozen) soil. Frozen moisture in the soil is not accessible to roots, but it's not going to hurt them, and without enough moisture, plants won't be able to tolerate cold temperatures with the natural "antifreeze" made in their tissues.

If you aren't able to plant them soon, consider protecting them more effectively by heeling them in. "Heeling-in" is simply planting a potted plant without removing the container -- it adds volume around the root mass to slow or reduce freeze-thaw cycles and keep the roots more insulated. Since the plant is still potted, it will be easy to pull up when you're ready to move and plant them in their final location. However, since there is still only very porous potting mix surrounding the roots, potted plants that are heeled-in still need regular checking for moisture. Evergreens (especially conifers like spruce) do not show obvious signs of getting dehydrated like deciduous plants do, so you won't necessarily know until it's too late if desiccation damage is occurring. (Winter cold and drought damage for conifer foliage usually doesn't manifest until sometime in spring.) If a location in the ground isn't available for heeling-in the pots, you can build a mound or berm to "plant" them into instead, either with soil, bark mulch, wood chips, or even (unbroken) bales of straw.

Nurseries in Maryland often start receiving new hardy tree and shrub inventory as early as late February. As an example, I have seen fruit trees in 7 gallon pots shipped from a local supplier that time of year arrive with completely frozen containers (even with snow on them, so the moisture in the pot was definitely ice). The plants were not damaged and they emerged from dormancy normally. Many nurseries having to overwinter hardy tree/shrub inventory either put them in protected greenhouses (unheated except to keep them from getting well below freezing) or keep pots sunk into the ground for insulation.

Miri

You're welcome!
The compost could be mixed into the soil where you plant, or added later as a top-dressing. Soil life moves it into deeper soil layers in that case over time, though generally once the soil is warmer. If you do apply it on the surface, try to mulch over it so its not prone to erosion (or serving as a weed seed bed) in the meantime.

Miri

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