What Wiki Said

I was wondering why the first day of Spring in Canada fell on March 20 (Drew posted about it on facebook). I thought he must have had his dates wrong and that all that snow had fogged his brain. Then I read on another northern hemisphere blog that Spring had just Sprung. So I googled it and found this. He was right and I was wrong (ahem)....and trust the Aussies and Kiwis to defy the natural occurence of the seasons and ignore the moment "when the sun passes through the equatorial plane"....and just pick a date.

When is the first day of Spring 2011 in Canada?

First Day of Spring is on March 20, 2011

Astronomical spring in the Northern Hemisphere as defined by the International Astronomical Union begins with the Vernal Equinox on March 20, 2011, at 7:21 p.m. EDT.

At the start of spring (spring equinox), day and night are approximately 12 hours long (at the equatorial plane) and the Sun is at the midpoint of the sky. Our north pole tilts towards the Sun.

First day of spring in the Northern Hemisphere

In general, the four seasons correspond to the relative position of the sun to the earth. Meteorological determination of spring is calculated according to when the sun passes through the equatorial plane. When going from winter to spring, the sun is moving north; as soon as the sun crosses the equator, we call it spring. (This applies to places north of the equator.)

First day of spring in the Southern Hemisphere

The "official" date of spring south of the equator (official is corresponding to the first day of fall in places north of the equator) would be around September 20/21, depending on when the sun crosses the equator.

Countries such as Australia and New Zealand, however, designate the first day of September as the official first day of spring (climatological counterpart). Preference between these two methods varies across Europe.

Many east Asian countries use lunar dates to determine the beginning of spring.

http://wiki.answers.com/Q/When_is_the_first_day_of_spring_2011