Do This On The Vernal and Autumnal Equinox

We do this at our house; stand an egg on end, but only on the vernal or autumnal equinox (approximately September 21st and March 21st, the first day of Autumn and Spring.) It just pops up there fine, but we have never been able to achieve it any other day of the year. And to make matters weirder, when we have left the egg standing all day, and even over night, it always falls over when the equinox passes. hmmmmm