It's usually recommended to plant after last frost and that's when I've planted mine. They don't tolerate a hard freeze but will usually sprout back after a light frost or freeze. They like warm days and cool nights and stop flowering during summer in hot summer areas.
We have such a varied climate here in Scotland so as a general year, I wait and plant my seeds just as soon as I see the weed seedlings apear, nature normally knows better than us impatient gardeners as Ive made the mistake of planting seeds too early many times before.
Good luck. WeeNel.