You need to know your last frost date.
Most seed packets will tell you when to start seeds.. Usually 4-6 weeks prior to setting out.
Count back from your last frost date.
Cold hardy plants can usually go out about a month before last frost date.
How do you find out what the average last frost date is?
We’re still getting frost. Heck, it’s snowing right now, even though the robins have returned and the bulbs are coming up.