Ah, finally. It sure took a lot of prodding to get this question asked. :-)

Although apparently many of my readers are unaware of it, the vernal equinox has become known as "Egg Balancing Day" in much of the world. Each year around March 20, schoolchildren (and others) take part in a bizarre ritual of standing dozens of eggs on end. They get their pictures in the local paper, and sometimes even appear on the evening news. (Seriously, you haven't seen this?)

This ritual centers on the vague belief that at the vernal equinox, the day when night and day are of equal length for everyone in the world from Santa Claus to emperor penguins [see the Answer Man Archive for a longer answer on the vernal equinox], there is a unique alignment of the sun's and Earth's gravitational fields which makes it easy to stand eggs on end.

The truth is, this is rubbish. Yes, you can stand an egg on its end on the vernal equinox, if you try hard enough. But you can do this on any other day of the year too! There's nothing exceptional about this particular day in terms of egg-standing ability (the alignment is repeated in the autumn, yet no one makes a big deal out of it then). The only reason more eggs stand on end on the vernal equinox is because more people are trying to stand them on end. The other 364 days a year, no one really thinks about it.

So how did this whole myth get started? It's surprisingly easy to trace. In 1945, Annalee Jacoby wrote an article for Life magazine about a phenomenon she witnessed in China, where a crowd in the city of Chungking stood eggs on end to celebrate Li Chun, the first day of spring on the Chinese solar calendar. (Apparently an old Chinese tradition said that eggs would balance especially well on the first day of spring, and no doubt egg-balancing would bring good luck or great riches to the balancer.) Jacoby's article was published in the March 19 issue, and started a minor egg-balancing craze in the U.S. to celebrate the first day of spring.

The funny thing is that Americans and Chinese had a different notion of the "first day of spring." In China, Li Chun was actually celebrated six weeks before the vernal equinox, but that part of the equation didn't cross the Pacific. Thus, true believers in the U.S. became convinced that eggs would balance only on (approx.) March 20, while true believers in China were certain it was only on (approx.) Feb. 4! See a contradiction here?

The egg-balancing tradition continued, and really took off in the '80s. As Phil Plait reports:

The biggest blooming of the virus happened on March 20, 1983, when Donna Henes, a self-proclaimed "artist and ritual-maker," got a hundred people in New York City to publicly stand eggs up at the vernal equinox. This event was covered by the New Yorker magazine, and the article was published in the April 4, 1983 issue. At 11:39 p.m. (the exact time of the equinox), Ms. Henes stood an egg up and announced "Spring is here."

Subsequent New York celebrations grew even more elaborate, further solidifying the connection between eggs and the vernal equinox, as science somehow got swept farther under the rug. I even remember learning about this amazing egg-balancing thing in school... where we were also taught the myth that drains flow one way in the Northern Hemisphere, and the other way in the Southern Hemisphere. (I've had a lot of un-learning to do.)

In recent years, most news organizations seem to have grasped that this is a social phenomenon, not a scientific one, and any coverage of egg-standing events at least ends with, "Is this really true? Probably not." Hey, it's still fun, though!

Sources: Phil Plait's Bad Astronomy, BBC h2g2, HighBeam Encyclopedia