Soil temperatures are going to be the overriding factor. Callas are a summer flowering bulb in your area and will only start producing growth when soil temps (and indirectly, air temps) are sufficiently warm.

You may want to use this advice from the Pacific Calla website: In other areas (zones lower than 8-10), they can be planted when temperatures rest higher than 55 degrees F (below 55 degrees, calla lilies stop growing). Just make sure that there is no danger of frost or of temperatures dropping below 55 degrees in the first 12 weeks after planting.

They are not going to be inground winter hardy for you, so plan accordingly.

Here is a link that might be useful: Pacific Callas

Forgot to add that despite despite the name calla "lily", these plants/bulbs are not lilies at all but members of the aroid family, which includes plants like skunk cabbage, anthuriums, elephant ears and a bunch of different houseplants.