The Vernal Equinox

I was thinking today (as it is the first day of spring) how we identify dates. The calendar day flips over every evening at midnight. This makes sense. Noon is midday, the halfway point, so we take twelve hours in either direction to determine the calendar day. But the equinoxes and solstices are determined by the length of day versus the length of night. Today (?) for example, there is twelve hours of daylight and twelve hours of night. So when, then, is the first day of spring? Well, wouldn’t it be the first day where there are twelve hours of day and night? So do we start at sunrise or sunset? Either way, it is pretty clear that it cannot line up exactly with a calendar day, because the calendar days start and end at midnight. I guess my morning musings reinforced the experience of calendars disagreeing when the first day of any season is. The fact is, calendar days are sort of unnatural when you consider there is dark-light-dark in every day. It seems to me that a more natural way would be sunrise to sunrise. Ah, but what are you going to do?