Complex sentence: Chhath, also called Dala Chhath, is an ancient and major festival in Bihar, and is celebrated twice a year: once in the summers, called the "Chaiti Chhath", and once around a week after Deepawali, called the "Kartik Chhath".
Split into simpler sentences: Chhath, also called Dala Chhath, is an ancient and major festival in Bihar. It is celebrated twice a year: once in the summer, called the "Chaiti Chhath", and once about a week after Deepawali, called the "Kartik Chhath".