See, here's the thing: If you care about the environment in a reasonable way -- not like those nut-case people who cry when a tree is cut down -- you act on that concern daily. You walk to the corner coffee shop, rather than drive the one mile distance. You'll ride a bike down to the tavern for a beer, rather than drive.
You spend your money on goods and services that are environmentally friendly. Buy from companies that make wise use of resources. Use products that don't involve harsh chemicals (ever consider using vinegar to clean your windows, rather than Windex?)
You find ways to reuse items, rather than purchase new (my dog's "camping water dish" is the bottom 1/3 of an old gallon milk jug.) You make do with what you have. You turn off the lights, or dial down the thermostat a degree or two.
It's not a hard way to be... small things help, and you don't get lumped into the eco-crazy nutcase group.
But you do these things daily. Not on one day of the year, but all 365 of 'em. And thus, you render Earth Day pointless.
Update: Related thought by John Andrews.