I usually do this in the spring when the temperatures are good for my area. Start them out as seeds, then into seedlings and then transplanted out into the vegetable garden. I have discovered that lately there is a new business afoot in my town: importing seedlings from warmer climes just as the weather warms up to plant seeds here.
So for the past two years I have gotten a super head start by purchasing a few of these already developed plants, while mine are still in their seed stage below ground. Gets me food much earlier. I saved a few of the trays to reuse this year for my own seedlings. I bought some potting soil and planted, expecting to have the same results as the commercial ones that I was buying from the same trays.
Trouble is, mine don't really grow very well, even in the warm sunshine. What exactly am I missing about these trays? My plants grow much faster in regular seedbeds. Must be the mix they use commercially... any help welcome.