Well, let me start by saying that we're very much amateur gardeners. But the directions said to plant the seeds "as soon as you can."
We had a warm spell, and have beautiful a beautiful garden area which has done fabulously for the past three years. The soil was perfectly loose/thawed when we planted. We had several inches of snow a couple of days later, but it slowly melted over the seeds. I figured it was perfect. Our temps have been in the 40-50s pretty steadily, with 60s a couple of days ago. I thought for sure, that they'd peek out then.
I know that our "zone" is one number higher than others at our latitude, because we have the warming effect of Lake Michigan (I think the surrounding area is 5, and we're considered 6).