The festival began about 600 years ago, when the Inca emperor invited everyone in his kingdom to Cusco for a giant party at the time of the winter solstice. (In the southern half of the world, winter is in June, not December.) The Inca wanted to thank Inti, the sun, for keeping the world bright and warm.
Today, Inti Raymi looks like a giant outdoor play. Hundreds of performers dress in colourful Inca costumes - feathered headdresses, gold-embroidered cloaks, bright red and yellow robes. They re-enact the ancient ceremony in front of crowds of more than 100,000 people. The whole city of Cusco joins in.
The main part of the celebration happens at a place called Sacsayhuam谩n, a huge old Inca fortress on a hill above the city. It is built of enormous stones, some of them as tall as a person. Inti Raymi has been performed there every year since 1944, when the festival was brought back to life after centuries.
Children in Cusco often learn Inca dances at school for weeks before the festival. Many families travel from villages high in the Andes to take part. After the main ceremony, there is music, dancing and food in the streets late into the night.