Why are some olive oils called "early harvest"?

Kosterina EVOO is always harvested early. “Early Harvest” is a designation given to extra virgin olive oils that are harvested early in the season - aka when the olives are still unripe (aka green!) and have the highest polyphenol content (and therefore health benefits); therefore, harvesting olives early is key to making the best quality extra virgin olive oil.

Of course, as you might have imagined, unripe olives harvested early yield less juice, which means more olives are required to make a bottle of EVOO, which is why authentic early harvest EVOO tends to be healthier and more flavorful (...but also more expensive).