Check out the University of Maryland Home and Garden Information Center for a vegetable calendar.

PS: Here's what they say about July:

# Perpetual spinach, Malabar spinach, and Swiss chard are good heat tolerant salad greens for Maryland. ÂGrand RapidsÂ, ÂRed SailsÂ, ÂDeer TongueÂ, ÂSloboltÂ, ÂBronze Arrowhead and all oak-leaf type lettuces are some of the heat tolerant looseleaf lettuce varieties that can be sown now. Keep them fertilized and watered and covered with a shade cloth, or floating row cover. Grow them in containers in the shade if possible.

# Its time to begin thinking of fall vegetables, seed for fall crops of broccoli and cauliflower should be sown in containers by the 3rd to 4th week in July. Late crops of squash, beans and cucumbers can be direct sown through the end of July.

Here is a link that might be useful: Maryland Extension - July

I haven't done this yet, but I plan to do it this year. Here's the general idea:

Figure out your average first frost date. Figure out the days to maturity for the things you want to grow. Subtract the latter from the former, then subtract a bit more time to allow things to actually produce for a while once they get started.

If you're worried that your "average" first frost date might be unreliable, and you might get a frost a lot earlier, either plan to cover your crops as needed, or subtract a bit more time.

If the things you want to grow can handle a light frost (many cool-weather crops, which are classics for Fall planting, can do this), you don't need to subtract so much to protect yourself from frost. Remember that it's usually a while between "first frost" and "dipping below freezing all the time."

All of this means that you'll likely be planting at the height of Summer. If the things you want to grow don't germinate well when it's too hot (definitely a possibility), then you might need to start things indoors and then transplant them later, just like you would when it's too cold in the early Spring.

I've also got a book called Small Space, Big Harvest (which is now out of print, sadly) which has in it, among many other useful things, some charts about when to plant different crops depending on your average first frost date. Perhaps such things are also available online.

So...clear as mud? :-)