You will see this, either on seed packets themselves, or in catalogs. What this means is as soon as the ground in unfrozen, you can plant these seeds. The ground being unfrozen means, when you can dig down, with a shovel or hand trowel, down about 8-10". So if the first few inches are unfrozen, but anything below still is.. it's still too early. There are lots of vegetables that you can plant way before the frost date. Our frost date here in New Haven CT is May 15th. But about now, you can plant peas, lettuce, spinach, kale, collards, chard, among others. Remember, even though the air temperature might dip down under freezing, the soil once unfrozen, won't freeze again. This is for planting seeds ONLY! No little seedling transplants will be able to handle the uneven temperatures, but seeds in the soil can. So, even though it's still winter, time to plant in the garden is NOW!