I started my sweet peas & snow peas about 3 weeks ago. Even tho it's been 30's at night and 40's during the day quite a bit (With a rare few days in the 60's), they seem to be loving it. I know the back of the seed packet says to plant them several weeks BEFORE your last 10% frost date.
Some gardening books simply say "Plant snow and sugar peas as soon as the ground can be worked", in other words when enough snow has melted and the ground is no longer a hard, frozen crust.
Carrots, Parsnips, English Peas (snow, snap), Cabbage, Broccoli, Cauliflower, Brussels Sprouts love cool weather. Parsnips actually improve in flavor substantially from a few freezes. In Houston, people have been known to bring container-grown parsnips into a walk in freezer for a day or two to encourage this flavor.