Start peppers and tomatoes three weeks before your climate zones last frost-free date. Here in Climate Zone 6b, that means my average frost date is April 15. (Such an auspicious day, anyway.) That means that I can start tomatoes and peppers in the end of March or beginning of April.
Well, I can plan, anyway.
Y'all folks who live in the southern tips of Texas, Florida, and Louisiana: start planting now! (You lucky ducks.)