If you're in the southern half of the country I wouldn't recommend putting them outside until the end of September. If you're more north you have 2 seasons and can do a winter crop with reasonable results. The two main considerations when deciding when to plant is hours of light and temperature. In the southern half of the country it can get too cold during September, and there is less than 13 hours of light (twilight to twilight) until the end of the month.
I suggest starting your plant inside under light until they've developed a good root system then plant them outside. They're most vulnerable during the first few weeks.