I usually try to shoot for mid to late April for my onion plants ( between the 18th and 25th) and late April for my potatoes. Even with that I had frost nip my potato plants. They came back just fine. Onions will take temps down to the twenties. Watch your soil conditions, you don't want to be planting in cold muck. Other people will tell you they plant their potatoes on Good Friday, but having done that a time or two to satisfy my DH we found that they didn't sprout any earlier than some later planted ones.