My seed directions say to plant when the soil temp is 65 degrees.
Certified GKH Gardening Expert
Technically, they expect you to go out there and use a probe thermometer. But most gardeners don't have those hanging around the garden shed. :)
An easier method is to simply stick your hand in the soil. If you can keep it comfortably down in the soil for a full minute, the soil is 60-65F. If you can't, it is colder than that.