Today marks Mandela Day - also known as Nelson Mandela International Day. It is held every year on 18 July.
It is on this day because this marks the birthday of Nelson Mandela - the South African anti-apartheid leader. The special day was created by the United Nations (UN) and honours Mandela's achievements. It has been celebrated every year since 2009.
Mr Mandela, who died in 2013 aged 95, was South Africa's first black president after spending 27 years in jail, and is remembered as one of the greatest leaders the world has ever seen.
The UN says: "Everyone has the ability and the responsibility to change the world for the better. Mandela Day is an occasion for all to take action and inspire change."
Newsround explains why Mandela is such a legend.
When Nelson Mandela was a young man, white and black people in South Africa lived separate lives under a system called apartheid.
White people, who were a small part of the population, were in charge of the country.
It was illegal for black people to use the same schools, hospitals and even beaches as white people. Conditions in whites-only schools and hospitals were much better.
Black people were also denied basic rights, like being allowed to vote in elections.
But Nelson Mandela believed that everybody should be treated equally.
Mandela joined a political party called the African National Congress (ANC) and later co-founded the ANC Youth League, leading protests against apartheid.
Sometimes the demonstrations turned violent and in 1964, Mandela was sentenced to life in prison on Robben Island.
While Mandela was in prison, photos of him were banned and it was even illegal to quote him in public.
But people from all over the world campaigned for his release.
Songs were written and big concerts were held in protest.
Finally, in 1990, the South African President FW de Klerk - a white man - allowed him to go free.
Mandela had spent 27 years in jail and was greeted as a hero on his release.
He is famous for promoting a message of forgiveness and equality.
Apartheid was abolished in 1991, and three years later, South Africa held its first elections in which black people, as well as white, were allowed to vote.
Nelson Mandela was elected president and set about trying to bring people of different races together.
In 1993, he was awarded the Nobel Peace Prize - the highest honour of its kind - for his work.
In 1995, South Africa held its first major sports competition - the Rugby World Cup.
Mandela gave his support to the South African team, made up mostly of white men, which helped to unite the country.
He became one of the world's most popular leaders, with politicians and celebrities queuing up to have their photo taken with him.
Even though Nelson Mandela helped to make South Africa a fairer place, the country still has lots of problems today, including poverty, violent crime and disease.
But he is remembered around the world for his message of peace and unity.