When you plant depends on the species and the soil temperature. Google the kind of plant you want to grow and see what it requires. I started tomato and pepper seeds 10 days ago. I keep the soil temperature at 85f. The tomatoes are very happy at that temp, but only one of the peppers has sprouted so far.
The length of day is important. Onions won't mature if they don't have the correct number of hours of daylight.
Day and night temperatures are also important. Tomatoes will grow, but they won't set fruits if the night time temperature drops below 50f.
I suggest doing a lot of research on the plants you want to grow.
!0 days after planting the San Marzano, Jersey Giant, and yellow tomatoes are up and growing, but only one pepper, a King Arthur bell pepper, has shown its head. Habaneros are notorious for sprouting slowly, even at 80-90 degrees.