VA Tech has a good publication on fall vegetables. Link is below. The general rule is to count backwards from your average first frost date by the number of days it takes to get from seed to harvest for each crop. The package should tell you or check an internet source for the information. Some crops will withstand light frost, some can tolerate light freezing. Most greens are actually better after frost.

Sandy

Here is a link that might be useful: Fall Vegetable Gardening