The first International Day of Remembrance of the Victims of Slavery and the Transatlantic Slave Trade took place in 2007, to coincide with the 200th anniversary of the abolition of the slave trade.
The UN-designated day honours the millions of men, women and children who were victims of the transatlantic slave trade, which the UN describes as "among the worst violations of human rights in the history of humanity".
Up to 28 million Africans were enslaved during the course of the trade, writes the BBC, which began in the 16th century and spanned over four hundred years.
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The notorious route saw Africans taken across the Atlantic by Europeans, mainly to colonies in North America, South America and the West Indies.
It is estimated that as many as one in five of them died during the journey due to cramped, unsanitary and brutal conditions.
While 25 March is a day for remembrance, it is also used to raise awareness about modern forms of slavery, which affects 45.8 million people worldwide, according to the Global Slavery Index.
What was Britain’s role in the transatlantic slave trade?
Britain, as well as its trading neighbours Spain, Portugal, the Netherlands and France, gained significantly through slavery through the "triangular trade", says Liverpool's International Slavery Museum.
Manufactured goods, including guns, alcohol, cloth and iron, were shipped to Africa and exchanged for slaves. The ships then sailed across the Atlantic and the slaves were auctioned off to the highest bidder, for whom they were forced to work in gruelling agricultural industries, such as sugar, cotton, tobacco and coffee – which were shipped back to Europe.
Britain reaped the benefits, studies show. Demand for factory-made goods increased, entire new industries were created to process the imported raw materials and profits were invested in British industry.
From the country house-owning elite to the enterprising merchant classes and even the poverty-stricken textile workers, "almost all were implicated".
The Legacies of British Slave-Ownership Project at University College London catalogues 20,000 British slave owners, taking the information from government records of compensation paid to them for the "loss of human property" when slavery was abolished in 1833.
The money raised in compensation – equivalent to £17bn today - was the country's largest state-sponsored pay-out before the banking crisis in 2008, says the BBC.
Britain’s role in abolishing slavery is the wrong memory to extract from this dark period of history, The Observer said last year, and has led to a "sanitised version" of the role the country played.
"Our political leaders hardly make reference to the dominant role this country played in the global slave trade," said the paper.
"Britain's true role in the slave trade must feature more prominently in our collective history and our national consciousness… Too often, Britain is portrayed as a benevolent benefactor rather than a country with a burden of debt to some of the world's poorest nations."
How is the day commemorated?
Each year, the UN runs a programme of events inspired by a different theme, from the resistance movement to the role of women.
Film screenings, musical performances and culinary events are being staged in New York, where the UN is headquartered.
A global student video conference will discuss the legacy of slavery and how best to overcome modern forms of racism.
To mark this year's theme of recognising the worldwide contribution of people of African descent, the International Slavery Museum in Liverpool has produced an exhibition to champion the activists who paved the way for black civil rights.
The Legacy of Black Achievement is on display in New York and will return to Merseyside later this year.
