Are you referring to starting seeds, taking cuttings to root or buying and planting store bought plants?
You would probably want to buy them when you see them in the garden centers or other stores because they are not a plant that is usually restocked later in the season, then plant them in the garden after the date of your average last frost.
We start ours from seed in early to mid February in the house. We are supposed to be able to plant them out in late May, average last frost here is May 15, but even planting them mid June they usually die here due to cold nights. If you start in early to mid Feb be forwarned they can get really big by mid to late May, many of ours get to four and five feet tall by then. They look more like small trees than tomato plants.... lol...
You might want to prune the top. That will throw more energy into the rest of the plant rather than it just growing tall and spindly. You really should just start yours later. Or plant when they appear on store shelves, the plants are always better that those I start. Let them do the hard work of getting them started.
If you want store bought tomatoes then plant them somewhere in the middle of May up to the beginning of June even and everything should be fine. Tomatoes are very gentle plants and it's unfortunately easy to harm them.
