From the Los Angeles Times:
Grapefruit may be the most misunderstood of California's fruits. Excellent locally grown examples are available year-round at farmers markets, but it's also easy to fall for fruit — often similar-looking and grown just a few miles away — that may be ludicrously sour or overmature. Choosing quality fruits depends on understanding the calendar of varieties and growing areas, which may seem inscrutable to the uninitiated but is easy to learn.
It's therefore no surprise that more than half of the roughly 9,000 acres of grapefruit grown in California are in the torrid Coachella and Imperial deserts. Commercial growers start picking in November, when the fruit typically needs to be "sweated" with ethylene, a natural ripening gas, to remove its greenish tinge.