Just now, at the end of July (and the Jewish Month of Tammuz,) are the grapes beginning to be edible. It doesn't pay to pick them early, because they just don't taste good and will not ripen off of the vine.
Our grapes are totally organic, and the only water they get is rain. (The first few years after planting we had watering system, but after it deteriorated, the rain sufficed.) That makes them extra sweet. A neighbor who once picked a nice quantity from them to make wine for both our families, is thinking of bringing a net to cover them from the birds and wasps. Then he'll try making wine from them, again.
I don't know the technical wine mavin term, but a sign of ripeness for green grapes is when the grapes look rather translucent, rather than waxy. The red/dark grapes must be dark to be ripe and sweet.