How will we know when our concord grapes are ready for harvesting? Is there a a specific set of dates we should be looking at for harvest?
Although color change is important in determining when to harvest grapes, it should not be the only consideration. Most berries change from green to blue, red or white (depending on the cultivar) as they approach maturity. But most grape cultivars color up long before they flavor up. When fully ripe, the natural bloom or whitish coating on the berries should become more noticeable. The color of the seeds changes from green to brown.
One of the other factors to consider in determining harvest time is the size and firmness of the berry. Most grapes should become slightly less firm to the touch. The best telltale sign of ripeness is the grape's sweetness, so the most reliable method for homeowners to test this is to taste them.
This article may help too: http://homeguides.sfgate.com/time-pick-concord-grapes-flavor-36264.html