Let’s be clear: there is no "official" start date. That is the most un-New York answer possible. We make our own rules. Some stores in SoHo had their lights up on November 2nd. Not because it makes sense, but because someone had to be first.
There’s a legend (which is probably true) that a bar on the Lower East Side keeps their Christmas lights up all year. They just turn them off in July and call it "mood lighting." Is it ridiculous? Yes. Does it save them the hassle of hanging them again? You bet your winter hat it does.
So, when do the lights go up? The answer is: whenever the city’s collective caffeine addiction hits its peak—usually sometime between the first morning frost and the last leaf falling. Just look up. If you see a man in a puffy coat cursing at a ladder, it’s officially Christmas in New York.