You probably know that today is the Autumnal (in the Northern hemisphere) Equinox, the end of Summer and the start of Autumn. In fact, it's right about now, 5:44pm ET. I thought it was odd that the "day and night of equal length" was at a particular moment, until I learned that - despite the name - the Equinox is actually not quite that.
Of many complicated sounding definitions, I think the simplest is that the equinox is when the Northern and Southern hemispheres are equally illuminated by the sun, which is also when the sun is directly overhead at the equator. Think of the September equinox as when the Northern hemisphere (entering Autumn) starts to get less light than the Southern (entering Spring).
So why isn't that also when the day and night are equal? Well, it's approximately true, but since the sun is really really big and we measure sunrise when the top first appears and sunset when the top finally disappears (and also because the atmosphere refracts sunlight), the 12 hour day and 12 hour night is still a few days away, and may not be at precisely 6am/6pm, depending on your local timezone and where you are located in that timezone. More turgid explainering over at the HuffPo, with video.
All that said, I like to think of the equinox as a passage from one season to another through some kind of tipping point. It's a little more interesting that just a 12-hour day.