Inti Raymi, or the Festival of the Sun, takes place June 24 at the Sacsaywaman fort/ruins.
The all day event, which marks the Winter Solstice, is well scripted and includes the sacrifice of two llamas (staged). An actor will play the Inca ruler who will tell the authorities in Cusco’s main square to govern well and at dusk announces the end of the day’s ceremonies and a party starts.
You can buy tickets for reserved seats through any tourist agency in Cusco, although you are welcome to sit on the lawn with the locals for free. Hotel reservations should be made months in advance.