In the United States we don't have a strong May Day celebration because the May day observance was discouraged by the founding Puritans. But in France the May Tree became the "Tree of Liberty" and became the symbol of the French Revolution. This, in turn, became the day of the workers' festivals in Europe.
So gradually the pagan Spring celebrations were co-opted by various religious and political groups...
But first and foremost May Day is a celebration of Spring renewal and perhaps that is what our Earth Day is now becoming.... a re-invention of our lost Springtime celebrations.
Long Live May Day, the Maypole, the May Queen and the celebration of new life!