Only 20 million people celebrated the first Earth Day on April 22, 1970, according to the EPA. Wikipedia's succinct history includes the criticism, from "bright green" advocates, that Earth Day actually marginalizes environmental sustainability.
I disagree. Today the movement has spread worldwide and is a powerful marketing theme for many organizations, but that only adds to its message and meaning. Since 1970, an entire generation of youngsters has grown up and begun families of their own, living the sustainability ideals of Earth Day every day and passing those ideals to their children.
The US government has an Earth Day portal with links to all kinds of volunteer opportunities, suggestions for classroom activities, information about environmental awareness and action, and more. More links and suggestions at Earth Day Network (where this image comes from). Even NASA celebrates Earth Day.
Just a few of the many environmental and conservation groups carrying the Earth Day message are: the Wilderness Society, the Sierra Club, World Wildlife Federation, and more. Museums, schools, community groups, businesses are picking up the theme. In Second Life, nonprofits and their avatars will be holding Earth Day celebrations. Twitter has #earthday entries, of course (search for latest here).
Happy Earth Day and many, many more.