Well we celebrate world environment day, to make awareness amongst the people, to make them understand the violators of nature, how its getting worse, day by day. This year it was celebrated on June 5. All this is Done by the United Nations (:
We celebrate World Environment Day because it is a day that reminds us how environment and trees are important for us and how much they help us and so its a day when we can thank them and grow them and also protect them.  So we should celebrate this day not only one day ,i think every day.
And also on this day people come together and think new ideas to protect environment.