An event every year that begins at 12:00am on of September, repeating indefinitely
16 September every year: Remembering the Montreal Protocol to protect the ozone layer.
The United Nations General Assembly proclaimed 16 September the International Day for the Preservation of the Ozone Layer, commemorating the date, in 1987, on which the Montreal Protocol on Substances that Deplete the Ozone Layer was signed.
Some good news! The UN says that the Ozone layer is ‘showing signs of recovery’ (BBC News, 11 September 2014)
This YouTube clip has a bit of background information on the Ozone Layer and the Montreal Protocol: