The UN has a long list of "observance days", and weeks, and years. It is mesmerising. In addition to International Mother Earth Day, the International Day for Biological Diversity, World Refugee Day, World Habitat Day, and World Oceans Day, which seem ok, sort of, we have the International Jazz Day, World Teachers' Day (take a stand for teachers), World Post Day, and World Tourism Day, which somehow seem oddd. The next one is on Sunday (4th): World Cancer day.
Curiously, some days are 'day-free', whilst others have multiple 'days': there are 5 on the 21st March, for example.
The longest day is the International Day for the Right to the Truth Concerning Gross Human Rights Violations and for the Dignity of Victims, and the shortest is the Day of Vesak. Whilst there are so many missing 'days', the most obvious is the UN Day of UN Days. My birthday is currently free. Go on, ...