David's Celluloid-stumper touched off a spirited discussion about tomorrow's Autumnal Equinox. On that day, and on its Vernal counterpart in March, you can stand an egg upright.
This is true. The ability to balance an egg on these days is often attributed to day and night being equal in length. However, you can also balance an egg on the Summer Solstice (the longest day of the year), the Winter Solstice (the shortest), my birthday, your birthday, Super Bowl Sunday and Viking Day.
Bad Astronomy has a thorough debunking of this myth, which links to the above photo of Dr. Phil Plait from October 1998.
"The little-appreciated fact that most eggshells have small bumps on them makes this seemingly impossible task achievable. Although, during an equinox, every place on Earth experiences an equal length day and night (12 hours each), this fact has no practical effect on egg stability."
*Or right now. Grab a carton!