Wait until the danger of frost has passed and the weather has settled. Use the last average frost date in your area as a guide. For example, in Burlington, VT, the average last frost date is mid May.
So, we traditionally set out our tomato transplants around Memorial Day. About a week before transplanting your seedlings, start hardening them off by gradually exposing them to outdoor conditions. Place the pots in a protected spot outdoors (partly shaded, out of the wind) for a few hours each day, bringing them in at night. Gradually expose them to more and more sunshine and wind. (A cold frame is a great place to harden off plants.)
Once you plant your seedlings in the garden, keep an eye on the forecast. Every year is different and a late cold snap can stunt or kill plants. Always be prepared to cover plants with a row cover, such as our Garden Quilt Cover, if a late cold snap threatens. Eager to get an earlier start? With Tomato Teepees, you can plant a few weeks earlier.