List of WWD's so far: http://www.unwater.org/worldwaterday/about-world-water-day/past-world-water-days/en/
https://twitter.com/Water_Decade
http://www.un.org/waterforlifedecade/
Several resources on http://www.unwater.org/worldwaterday/campaign-materials/documents-and-information-resources/en/