April: Energy, Environment, Science and Technology: Earth Day
Each year, April 22 is internationally recognized and celebrated as "Earth Day."

What Is Earth Day?
Earth Day, April 22, is the annual U.S. celebration of the environment and a time for Americans to assess the work still needed to protect the natural gifts of our planet. Earth Day has no central organizing force behind it, though several nongovernmental organizations work to keep track of the thousands of local events in schools and parks that mark the day.

http://elibraryusa.state.gov/apr_theme.html
Additional Earth Day Links

Earth Day Network
U.S. Environmental Protection Agency
U.S. Department of Energy: Office of Energy Efficiency and Renewable Energy
U.S. Department of Energy: Energy Blog
PBS (U.S. television): EcoInvestigators
PBS: Journey to Planet Earth - Plan B: Mobilizing to Save Civilization (Video)
History.com: Earth Day
History.com: 10 Earth Day Videos
History.com: The First Earth Day