World Water Monitoring Day is observed on September 18 every year. This observance was established in 2003 by America's Clean Water Foundation to raise public awareness of the issues of pollution of water bodies.
Astronomy Day is annually celebrated in Armenia on September 18. This observance was established by the government of Armenia to commemorate birthday of Victor Ambartsumian, an outstanding Armenian scientist, founder of theoretical astrophysics and national hero in Armenia.
Clean Up the World is a global campaign that is annually held on the third weekend in September. The campaign lasts for three days, starting on Friday, and it involves an estimated 35 million people in 130 countries around the world.