What vintage actually refers to is wine produced from a particular harvest. When a wine is labeled with a "vintage date" it is in reference to the year in which the grapes were harvested, but not in which they were necessarily grown.
In the Northern Hemisphere our growing season exists within a single calendar year, beginning some time from late March to early May, and ending some time from August to early November, depending on the local climate. A vintage date of 2015 would signify that the grapes were grown and harvested in that year.
One deviation from this case is true ice wines. The grapes for these wines need to be harvested when it is the appropriate sub-freezing temperature and often that does not occur until mid-Winter. Hazlitt 1852 Vineyards' ice wines are most often harvested in the January that follows the growing season. While the grapes for our current 2014 Vidal Ice Wine were harvested in January of 2014, they were actually grown during the 2013 season.
In the Southern Hemisphere things are different. The growing season there begins in our Fall (their Spring) and the grapes are harvested in our Spring (their Fall). Therefore, a Sauvignon Blanc from New Zealand that is labeled as a 2016 had its grapes start their lives in 2015 but were not picked and processed until 2016.
There are winemakers that travel the world and do multiple vintages in a year, starting in the Southern Hemisphere and then repeating the process in the Northern Hemisphere.
Often times there are multiple vintages in the winery at a given time. Currently at Hazlitt we have the 2017 vintage in tanks in the cellars. This vintage includes both white and red wines produced from grapes harvested in the Fall of 2017. These wines are young and in some cases finishing fermentation. The whites will be clarified and bottled later this year.
In the barrel room we have the 2016 vintage of red wines. These wines will sit in barrels for some period of time between one and one and a half years. This time allows the small tannins extracted during fermentation the opportunity to polymerize and soften the harsh mouthfeel that young red wines possess.
Upon the 2016 reds reaching the desired point of maturation, the wine will be pumped out of barrels and then replaced with the 2017 reds to begin the process again. The reds will not be bottled until 2019.