International Ozone Day is celebrated on

The United Nations designates International Observance Days to mark significant environmental treaties, and the specific date chosen is usually the anniversary of the treaty's signing rather than an arbitrary calendar date.

The Montreal Protocol on Substances that Deplete the Ozone Layer was signed on 16 September 1987, committing nations to phase out ozone-depleting substances such as CFCs. In 1994, the UN General Assembly (Resolution 49/114) proclaimed 16 September as the International Day for the Preservation of the Ozone Layer, directly commemorating this signing date.

This is distinct from other environment-related observances that fall on different dates for different reasons: 22 April is Earth Day, a general ecological-awareness day unconnected to the ozone treaty, and 7 December is International Civil Aviation Day, marking the 1944 Chicago Convention. 30 March carries no UN designation tied to ozone protection. Only 16 September ties directly to the Montreal Protocol anniversary.