When the soil temperature in your area reaches the 50degree or more mark at least 3' down it is at prime time to plant the corn. But also depth of planing has a big factor on how well the crop will grow. Plant seeds 1 1/2 to 2 inches below the surface for a good root bed to take.
Corn needs to be planted in April or May when the ground has thawed, and the threat of frost has passed. There is an old saying, knee high by the Fourth of July. Corn should be at least knee high by July 4th in order to be ready to bear corn at the proper time of year.