I think whoever started it intentionally planned for it to be in the spring, when we're all so grateful to see green, and feel warmth, and be outside.
My sister reminded me of a very appropriate Scripture in her Facebook status:
"The earth is the Lord's, and everything in it." I Corinthians 10:26 (New International Version)
And my cousin makes a great statement via Twitter/Facebook:
"Real conservatives protect what God loves: babies, the poor, and the earth."
However you mark this day, be sure to enjoy God's handiwork! And consider how we might all respect it.