The word chhath denotes the number 6 in Hindi & Nepali and the festival is celebrated on the sixth day of the Hindu lunar month of Kartik.
The word Chhath is also a compound of two words: “Chah” meaning “six stages”, and “Hath” refers to the science of Hath Yog (austerity). The word Chhath refers to the process of consciously obtaining the solar energy through six stages involving the methods of Hath Yog. “Hath” here refers to the austerities such as fasting and standing in water. (source -Wiki)