It was officially declared by the United Nations General Assembly in November 2009, with the first official UN celebration of it occurring on his July 18 birthday last year. There were other groups that celebrated it on July 18, 2009.
Here's a videotaped message from UN Secretary General Ban Ki-moon about today.
UN Mandela Day is basically an international call to action that celebrates the idea that each individual has the power to transform the world and make an impact.
They ask that people take 67 minutes out of their day, equivalent to the 67 years that Mandela fought for social justice. And to borrow from the statement that was issued for this day on behalf of Mandela, "do something to bring people around the world together to help fight poverty, promote peace and reconciliation."
Happy birthday Madiba, and let's get busy changing the world.