Typically the first day of fall -- my older brother maintained it was the day everything started to die - my mother's version was it was the day everything started to get colourful. Either way, I was still enough in the sign of the virgin to be able to truthfully maintain "Oui, maman - je suis encore vierge" ("of course I'm still a virgin, mom!") long after it was physically untrue. Bonus!!
And I still tell my kids that, too.