For a summer harvest, start seeds indoors six weeks before your last spring frost, and set out hardened-off seedlings when they're about four weeks old. (that's purely from the web)
Word on the farm is that planting mid-summer and harvesting in fall produces a sweeter, better tasting crop.
If you are into spring greens, and have not yet done so, look into Chard. Incredibly nutritious, easy to grow, doesn't bolt like spinach, and can be used raw or cooked in a variety of ways. (that's purely from me)