NGO (Non-Government Organization) Day began in 2010 and was initiated by Latvian-born Mārcis Liors Skadmanis. It originated in the Council of the Baltic Sea States and is based in Humbold University in Berlin Germany.
In 2014, it became World NGO day with the participation of the UNESCO, the European Commission, and many others in its first international conference in Helsinki, Finland.
World NGO Day aims to inspire people to become more actively involved within NGOs and encourage a greater symbiosis between NGOs and both the public and private sector. The universal concept of the World NGO Day is Celebrate, Commemorate and Collaborate the various NGOs around the world and the people behind them.
NGOs (Non-Government Organizations) are also called NPOs (Non-Profit Organizations), CSOs (Civil Society Organizations,) Charitable Organizations, Membership Organizations, Third Sector groups, or Citizen Organizations.
Summarized from Wikipedia and the NGO Day Official Website.

Disclaimer: The information on this website is accurate to the best of my knowledge at the time of writing. I make no guarantee as to its accuracy. Its purpose is to inform, educate, amuse, and raise awareness about causes and opportunities around the globe. I also encourage civil debate in the comments.