We sow tomato transplants twice here on our farm in central Virginia: once in March, for our earliest crops, and again in mid-April, for a second tomato crop that will start producing in August. Although many heirlooms will produce continuously until frost, we find that production sometimes slows when summer rains or disease-pressure take a toll on our plants. Sowing that second crop gives us the best quality fruit late in the summer and into early fall (and for harvesting underripe fruits to ripen in storage when frost threatens).
Multiple tomato crop successions spaced about a month apart can also help you grow more flavorful tomatoes. We find our tomatoes are sweetest and most flavorful when the weather has been dry when the fruits are ripening. You can get this effect by reducing or stopping irrigation altogether when your tomatoes start to ripen. The fruits will be more intensely flavorful, but the plants will likely stop producing sooner. Time your next succession to start producing just when your parched plants are ready to quit.
We recommend sowing your early-crop tomato transplants 5 to 6 weeks before your average last frost. You can also sow sooner to get a jump on the season, but you may need to pot up into larger containers 2 or more times, instead of just one, to prevent the young plants from becoming leggy and root-bound.