The decision to harvest a particular cultivar is one made after careful consideration.
Viticulturists and winemakers are constantly assessing the weather conditions, the flowering, the berry set, and the vineyard health as guiding indicators of when to harvest. These keenly trained eyes and palates are earned, from years of working with each individual vineyard, tasting how the site ripens its grapes, and trusting oneself to make the call to harvest.
The factor that still holds the most weight when deciding when to harvest is the taste of the grapes and the texture of the tannins from the skins.