By resolution 2994 (XXVII) of December 15, 1972, the General Assembly designated June 5 as World Environment Day, to deepen public awareness of the need to preserve and enhance the environment. That date was chosen because it was the opening day of the United Nations Conference on the Human Environment (Stockholm, 1972), which led to the establishment of the United Nations Environment Programme (UNEP). Twenty years later, the Assembly convened UNCED in Rio de Janeiro, where nations came together to take the decisions needed to rekindle the hopes of the 1972 Conference and to take up the challenges of a viable and equitable balance between environment and development and a sustainable future for the earth and its people.
Source: United Nations Website: Observations