The TRUE "Earth Day" is the vernal equinox, March 21

[Earth_First]

Earth Day on April 22 became an internationally observed event partly due in part to the ability to organize outdoor events in better weather...

[slackmaster]

1. Vernal equinox is not always on March 21

The date and time fo the equinoxes (and solstices) depend on what year and which time zone you're in.

http://scienceworld.wolfram.com/astronomy/VernalEquinox.html

Earth Day on April 22 became an internationally observed event partly due in part to the ability to organize outdoor events in better weather...

Makes sense. Here in San Diego it wouldn't make much difference one way or another because it hardly ever rains.