Complex sentence: The main festival of Pyan Gaun is Indra Jatra, when the people of Pyangaun perform a form of dance known as Kug pyakha.
Split into simpler sentences: The main festival of Pyan Gaun is Indra Jatra. During Indra Jatra, the people of Pyangaun perform a form of dance known as Kug pyakha.