Did any European witness an Inca religious festival?
Garcilaso de la Vega, a Spanish-Peruvian chronicler in the Viceroyalty of Peru(then Spanish-held) recounted several aspects of Incan life and tradition. His most famous works include Historia de la Florida and Comentarios Reales de los Incas the second of which is of presumably more interest to you as it details some of his experiences in Cuzco as a child.
I looked online and found this excerpt from the first part of the Comentarios which details the "Festival of the Sun".
Cristóbal de Molina, a young Spanish priest, witnesses in 1535 the Inca celebration of the maize harvest:
On a platform Indians were throwing meats into a great fire. At another place the Inca ordered llamas to be thrown for the poorer Indians to grab, and this caused great sport. Over 200 hundred girls came out of Cuzco, each with a large new pot of maize beer. They also offered to the sun many bales of a herb that the Indians chew and call coca, whose leaf is like myrtle.
There were many other ceremonies and sacrifices until, as the sun was disappearing from sight, they made a great act of reverence, raising their hands and worshipping it in the deepest humility. On the ninth day, when the festival was over, they brought out hand ploughs. The Inca took a plough and began to break the earth, and the rest of the lords did the same. Following their lead the entire kingdom did likewise. No Indian would have dared to break the earth until the Inca had done so, and none believed that the earth could produce unless the Inca broke it first.
Read more: COLOURFUL FESTIVAL.
Or, you can purchase an entire book about this : Account of the Fables and Rites of the Incas
Pizarro himself may have witnessed a minor religious festival in 1529, on his first trip to Peru; however, this is disputed.