Each year, the vernal equinox signals the beginning of spring (in the Northern hemisphere). So what determines the date of the vernal equinox?
Technically speaking, the vernal equinox occurs on the day when the Earth's axis (which is tilted by 23.5°) is pointed neither towards nor away from the Sun. The Sun will appear directly centered over the Earth's equator. Put simply, however, the length of the day and night are nearly equal, hence the term equinox. Therefore, after the vernal equinox, the days will be longer than the nights, signaling the beginning of spring. Typically the equinox occurs on March 20th or 21st.
Precession of the Equinoxes
The Earth's spin axis is not fixed in direction, however. It actually wobbles about its axis, much like a top. It takes our planet about 26,000 years to complete one "wobble". This has some interesting effects on the view from planet Earth. Not only will it eventually cause our beloved pole star, Polaris, to no longer remain stationary, but it also causes an effect known as the "precession of the equinoxes". This causes the point on the celestial sphere that the Sun occupies to move over the course of 26,000 years.
Lately, much ado has been made about this, as the Sun will soon appear to cross the galactic center on the winter solstice of 2012. This is pure superstition, however, and has no credible basis in real science (just see this post).
Contrary to popular belief, it is not any easier to balance on egg on the equinox than it is on any other day of the year. If you try long enough, you can balance an egg on its end any time of year. It just happens to be that more people try it on the equinox than any other time, and so the myth perpetuates.
The following table lists some upcoming equinoxes.