Think of the growth cycle for grapes as being similar to other fruits. An apple tree's cycle is: bud break in spring, leafing, flowering, little apples turn into big, sweet apples in the summer, and you pick them late summer to early autumn when they're fully ripe. Ripeness in apples means a good sugar /acid balance for eating or cooking. It's basically the same for grapes.
Here's a easy-to-understand explanation from the Napa Valley Vintners.
|Mid-July in Sonoma Valley. A heavy crop!|