The heat is pretty scary isn't it?
Mid-July through mid-August isn't too soon. They won't grow fast until conditions are right but they need those extra weeks with more sunlight.

Most of the garden standard crops sprout best around 70 degrees. If it is hotter or colder than that germination drops off. Sooooo the best plan would be to sprout your babies indoors and then move them outside once they have their first set of true leaves. Most of them handle the high heat just fine once they've sprouted. I would shelter them from harsh sun in the beginning (sometimes I take pruned branches from weed trees and bushes and cram them into the ground to provide shade or I place potted houseplants that are spending the summer outdoors where they will shade the babies the first month or so). The list of direct sown seed success in the winter garden is short, cool weather before hard freezes is just too short a season.

When the stores start carrying the seedings for cabbage and collards, that's the time! I planted cabbage last year that I literally harvested the night we got the 6 inch snow here in central NC. In the summer, the plants just stared at me--surviving, but not thriving. Then, when the weather broke, they took off growing! I have great luck with lettuce, but I can't get spinach to do a thing, I confess.