Vernal (Spring) Equinox

Spring officially arrives here in the Northern Hemisphere on Monday, March 20th. Most sources will tell you it is the day when the Sun passes directly over the Equator as it (the Sun) moves northwards. This is not true. The Sun does not move north or south with the seasons. It only appears to do so. In reality, the Earth’s North Pole tips towards or away form the Sun. While doing so, the Earth’s Equator moves under the Sun.

Having gotten that off my chest we have these sites and explanations.

From Infoplease:
The Rite of Spring

On March 20, 2006, at precisely 1:26 P.M. EST (18:26 Universal Time), the Sun will cross directly over the Earth's equator. This moment is known as the vernal equinox in the Northern Hemisphere. For the Southern Hemisphere, this is the moment of the autumnal equinox.

The date is significant in Christianity because Easter always falls on the first Sunday after the first full moon after the vernal equinox. It is also probably no coincidence that early Egyptians built the Great Sphinx so that it points directly toward the rising Sun on the day of the vernal equinox.

The first day of spring also marks the beginning of Nowruz, the Persian New Year. The celebration lasts 13 days and is rooted in the 3,000-year-old tradition of Zorastrianism.

(This article is duplicated at Fact Monster


Equinox and Solstice.com has this about the rituals associated with the First Day of Spring:

Rituals and invocations for abundance in the new crops being planted would often be held during the new moon closest to the Equinox (traditionally a good time to plant).
…
Even the latter day celebration (comparatively speaking) of Easter acknowledged the significance of the Vernal Equinox. The Council of Nice decreed in 325 A.D. that "Easter was to fall upon the first Sunday after the first full moon on or after the Vernal Equinox."

And Wikipedia contains TEXTthis explanation of why the day isn’t really equal to the night on the Equinox:
http://en.wikipedia.org/wiki/Equinox

In practice, at the equinox, the day is longer than the night. Commonly the day is defined as the period that sunlight may reach the ground in absence of local obstacles. This is firstly because the Sun does not appear as a single point of light, but as a disc. So when the middle of the Sun is still below the horizon, the topmost edge is already visible and sheds light. Furthermore, the atmosphere refracts light downwards, so even when the topmost edge of the Sun is really still below the horizon, its rays already reach around the horizon to the ground. These effects together make the day about 14 minutes longer than the night (at the equator, and more towards the poles). The real equality of day and night happens a few days towards the winter side of the equinox.