The World Ozone Day is celebrated on

The United Nations General Assembly designates specific calendar dates as international observance days to mark landmark environmental treaties and raise awareness of a specific issue; each date is fixed by the founding UN resolution, not by the day's general theme.

World Ozone Day — officially the International Day for the Preservation of the Ozone Layer — falls on 16 September, the anniversary of the 1987 signing of the Montreal Protocol on Substances that Deplete the Ozone Layer, the treaty that phased out CFCs and other ozone-depleting substances.

The other three offered dates are also real UN/global observances, but for different causes:

• 21 March is the International Day of Forests (World Forestry Day) — about forest conservation, not the ozone layer.

• 25 April is World Malaria Day — a public-health observance, unrelated to atmospheric chemistry.

• 5 June is World Environment Day — the UN's broadest general environmental observance, distinct from the ozone-specific day on 16 September.