Keti Koti means 'broken chains' in the Sranantango language (creole language spoken in Suriname, a former Dutch colony in South America which became independent in 1975).
The Keti Koti festival in Amsterdam celebrates the abolishment of slavery in the Dutch Antilles and Suriname, which formally took place on July 1, 1863.
Formally, because the slaves had to work a mandatory 10 years for free for their former owners, to compensate the owners for their loss.
The festival is held every year on July 1. It starts with a commemoration at the Slavery Monument in the Oosterpark, followed by an open air party. Details may vary per year, look at the website for that.
National Slavery Monument in Oosterpark
Slavery Commemoration Plaque @ Mayor's residence