In 1969 at a UNESCO Conference in San Francisco a day to honor the Earth and the concept of peace was proposed. First held in 1970, Earth Day is an annual event celebrated on April 22. It is considered the birth of the modern environmental movement. Since then many people participate worldwide on Earth Day in various events to demonstrate support for environmental protection.
Some resources to celebrate Earth Day (activities, lesson plans, lessons, videos, printables...) with your students:
http://www.earthday.org/ (Earth Day Network)