Tomatoes like warm soil and don't tolerate frost, so wait for….

Warm spring days and soil temperatures that are above 60° to plant. In cool climates you can use black plastic for 1-2 weeks before planting to help warm the soil. Remove plastic upon planting. Where weather is very warm, mulch tomatoes thickly after soil has warmed thoroughly. Fluctuations in soil moisture can be minimized with good mulching practices.