The point in the season where the grape clusters have mostly stopped growing and switch their energy over to ripening is called veraison. Physiologically, sugars start to build up and some of the acids start to break down. This process is visible particularly in red grapes as their color begins to change due to the development of anthocyanins and some of the other antioxidants that are said to be good for you.
Just like other fruits, different varieties of grapes ripen at different times. Most of our vineyard hasn’t quite hit veraison yet, but our earliest red grapes have. It’s always pretty cool to watch the color progress as everything ripens.