Winemakers and ampelographers (grapevine specialists) will tell you that it takes years of practice, tasting grapes and working the grapevines to know the optimal time to pick or harvest the grapes.

The ripening phase begins with the coloring (véraison) of berries and ends with the harvest.  During this time sugar is produced in the leaves and then stored in the fruit.  The more light and heat the leaves receive, the more sugar they will collect; the more sugar stored in the grapes, the higher the alcohol content of the wine. 

It takes about half an ounce (16 g) of sugar to create 1% alcohol in a liter of wine.

When sugar and the remaining acids are nicely balanced the grapes are considered to be ripe.  Most wines have an alcohol content of about 11 to 14 % by volume.  To reach this level, it is possible for the growers to calculate what sugar content the grapes will reach by harvest time.  The sugar level is reflected in the weight of the must (the juice of pressed grapes), which is measured with a refractometer or a hydrometer.

The different scales are based on the fact that grape must is heavier than water.  One milli-liter of water weighs 1 gram.  The excess weight of 1 ml of juice is the weight of the substances dissolved in it.  Since 90% of the solutes in grape juice are sugars, this is regarded as a pretty accurate means of determining the sugar level of must.

There are several scales for measuring must weight.  In France & Australia, must weight is measured in degrees of Baumé; in the Americas, in degrees of Brix; in Italy in degrees of Babo; in Germany, in degrees of Oechsle and in Austria they have their own scale (KMW), devised at Klosterneuburg.

The simplest measure of must weight is the Baumé scale.  It gives the potential alcohol content of the wine, assuming that all of the sugar is changed to alcohol by fermentation.  A must with 13°7¢ Baumé would therefore yield a wine with 13.7% alcohol by volume.  On the Brix scale this would be the equivalent of 24°7¢.

There you have it.  Now just in case you somehow get the honor of deciding when to start the harvest, you will know when to begin!