Similar astronomical conditions exist at the autumnal equinox, of course, but the first day of fall apparently doesn't generate the same eggy enthusiasm as the vernal variety. Any astronerd will tell you that vernal egg magic is a bunch of hoo-ha. Earth spins on its axis. Your gravitational orientation toward the sun is constantly changing. Also, the moon exerts far more gravitational force on Earth, and it's up there moving around like crazy on a completely different cycle than Earth's revolution around the sun.
Eggs are tough to balance for two reasons.
First, they must rest on a single point on their surface. Also, the yolk of an egg is much denser than the clear part, and the yolk sits high in the egg. The high center of gravity makes the egg want to fall over. Try balancing an ice pick, and you'll see what I mean. Still, with a little patience you should be able to balance an egg on March 20.