In general the harvest season depends on the region, and often the farmers wait as much as possible because ripen olives simply contain much more oil. However,

In general the harvest season depends on the region, and often the farmers wait as much as possible because ripen olives simply contain much more oil. However, less ripen olives produce more green tinted and strong flavored oil so some fancy brands sell "early harvest" olive oil, taken a few months before the regular harvest, as premium varieties.