The standard guideline for determining earliest planting date is when morning soil temperature at a 2 inch soil depth is 55º F or 50º F at a 6 inch soil depth. Planting before the soil temperature is warm enough for germination greatly increases the potential for stand failure.
Soil temperature may vary depending upon soil texture, slope, color and amount and type of crop residue. Thus, randomly measuring soil temperature with a thermometer within a field should provide a reliable indicator of desirable conditions for stand establishment.
From
this article.
I'd use the soil temp above historical frost dates and plant later rather than earlier unless you have an extremely short growing season.