Complex sentence: Durga Puja, the most important festival marked at the temple, is celebrated twice a year: in the months of Chaitra (April--May) and Ashwin (September--October).
Split into simpler sentences: Durga Puja, the most important festival of the temple is celebrate twice in a year. The first one is celebrated in the month of Chaitra (April--May) and the other in the month of Ashwin (September--October).