Robins in a snowstorm on the first day of Spring

A picture from my neighbor, Doug Dearhamer.
Spring begins when the sun passes through the First Point of Aries. This is where the ecliptic (the circle the sun traces out in the sky during the year) meets the celestial equator (just the Earth's equator projected into the sky). By definition, the First Point of Aries is also the starting point for calculations of right ascension and declination (like a "longitude" "latitude" way to find stars). Because of precession of the Earth's axis, the First Point of Aries is no longer in the constellation of Aries but moved into it's neighbor, Pisces. Sorry, probably more than you really wanted to know about the First Point of Aries but I think it's pretty cool stuff.