When is the world environment day celebrated?

Hint: World environment day is the United Nations initiative for promoting the conservation of the environment. It intends to create awareness regarding pollution and global warming. Every year, it is celebrated with a new theme.

Complete answer:
Every year on the 5th of June the World environment day is celebrated all around the globe. It was initiated in the year 1974 in the USA. Now, it has become a global platform that has active participation from over 143 countries. Major corporations, NGOs, governments, communities, and celebrities play their role in environment conservation according to the theme.
Two major global environmental issues are the greenhouse effect, which is warming up of the Earth and ozone depletion in the stratosphere. The enhanced greenhouse effect is due to the increased emission of certain gases like carbon dioxide, methane, nitrous oxide, and CFSs. Deforestation is also responsible for the increase in temperature of the Earth. Ozone in the stratosphere, which protects us from the harmful rays of the UV, is depleting fast due to the emission of CFCs, thus increasing the risk of skin cancers, mutation, and other disorders.

Note: -Pollutant which persists in the same form in which it is released in the environment is called a primary pollutant. Ex. $CO_2$, Plastic
-Pollutants that are formed from another pollutant due to a reaction are called secondary pollutants. Ex. Ozone, PAN (Peroxyacyl Nitrate)
-World biodiversity day is celebrated every year on 22nd May. India with 45,000 species of plants and twice as many species of animals are among the twelve mega biodiversities of the world.