Veraison (vay-ray-zon) is a French term that has been adopted into English literature regarding viticulture and grape growing. One simple definition of veraison is 'the change in colour of the grape berries'. Veraison represents the transition period from berry growth to berry ripening, and many changes in berry development occur during veraison.
The exact trigger of veraison is unknown, but veraison signifies that the grape seed is reaching maturity. However, seed maturity is unlikely the cause, as seedless grapes also go through veraison. In the northern hemisphere veraison, the ripening grapes begin to soften and change colour typically occurs anywhere from late June to mid-August, depending on the climate, and in the southern hemisphere from January to February.

 

During veraison tannins ripen and astringent malic acid begins to give way to softer tartaric acid. Leaves are often pruned, thinned at this time to give grapes more sunlight and air exposure. Mildew and disease are still a worry to viticulturists and spraying may continue in some vineyards. Sugar levels begin to rise in individual grapes after veraison. At this stage of the growing cycle, vines are rarely irrigated after veraison because this can and will dilute the grapes flavours.
A little winemaking 101 - as the seeds ripen so does the rest of the grape. In this process called veraison, the thick, hard green skins soften and change to either red-black (due to anthocyanins) or yellow-green (due to carotenoids) depending on the grape varietal. At this time of the growing season, sugars and berry volume increase dramatically and acidity decreases as the grapes reach the home stretch before harvest.
Overall, the grape approximately doubles in size (primarily water) between the beginning of the second growth period, veraison and then harvest. Also during the veraison period it is the start of each grape taking on the characteristics and personality of their specific varietal. Grapes are then usually harvested 40-60 days later, depending on the growing season, style of wine to be produced and the grape varietal.