Why does an egg stand during equinox?

During March 21 and september 22, i.e, vernal equinox and autumnal equinox, we can perfectly balance an egg. Can anyone tell me why?

You can perfectly balance an egg any day of the year. Here is the history behind the significance of the ritual on the equinoxes:

The Chinese are thought to have originated the practice of standing eggs on end during the equinox. Just as the equinox symbolically restores balance to the world by signalling its rebirth after a season of darkness, the equinox literally balances the day by dividing it into equal portions of darkness and light. If the symbol of fertility — eggs — could be balanced on end during a day equally divided between day and night, this was a sign that all nature was in harmony. That the balancing of eggs could be achieved on any day of the year was of no importance; what everyone wanted and needed was a familiar, reassuring ritual to demonstrate that all was right with the world.