Usually 6-8 weeks before the last frost in your area.
Find out your last frost date here:
Then count back 6 to 8 weeks. That is when to start your seeds.
I am in zone 9 and already have mine set out. I started my seeds in January.
I do mine in the spring
Look for the information on the back of your seed packet that says "days to germination". Count back from that amount of days and add two/three weeks. That will give you the time needed for planting, sprouting (germination) and growth before hardening off to plant outside.