World Water Day dates back to the 1992 United Nations Conference on Environment and Development where an international observance for water was recommended.The United Nations General Assembly responded by designating 22 March 1993 as the first World Water Day. It has been held annually since then.

Each year, UN-Water — the entity that coordinates the UN’s work on water and sanitation — sets a theme for World Water Day corresponding to a current or future challenge.


Source and futher info: http://www.unwater.org/worldwaterday/