You want to plant your seed starts 6-8 weeks before the last frost in your area. I’m in Zone 6 which covers a big chunk on the US. To find your zone click here: USDA Plant Hardiness Map. Usually the last frost is close to Easter. Roughly 6 weeks before Easter is Fat Tuesday. If you live in Zones 5-7, begin your seeds on Fat Tuesday.
Start your seeds in a greenhouse, our in a sunny spot in your house. Plant your seeds in biodegradable containers, or egg shells so you don’t have to transplant. Just place the seedling in the ground when spring arrives. If you start your seeds in an eggshell, the shell provides extra nutrients as the seed grows. Write in light pencil what you’ve planted on the shell so you won’t be surprised later when what you thought was a tomato is really a zucchini.