With all my tomatoes this year, I am trying to get an early start (easier for me in the climate here, though the temperatures can be unpredictable). The seeds I've started are already young seedlings, getting first true leaves. All are in three flats, in small pots, except for two (nameless) yellow grape tomatoes I started last fall, which are larger plants in larger pots.
They grow indoors when it is too cold outdoors, no greenhouse or polytunnel available to me. I move them outdoors daily when the weather is warm enough. After some reading, I've judged that above 10 C is the lowest safe temperature for moving them outdoors in the daytime, they have responded well. I've read that some can take it as low as 5 C, but I think I would only try that with cold-tolerant types.
I am hoping to have larger plants in pots by late March, moving plants on to 1 or 2 gallon nursery pots by mid April, and in the ground by late April or early May.