Wikipedia
Chhath ( Devanagari: छठ, छठी, छठ पर्व, छठ पुजा, डाला छठ, डाला पुजा, सूर्य षष्ठी) is an ancient Hindu Vedic festival dedicated to the Hindu Sun God, Surya and Chhathi Maiya (ancient Vedic Goddess ; Usha - wife of Sun God ). The Chhath Puja is performed in order to thank Surya for sustaining life on earth and to request the granting of certain wishes. This festival is observed by Nepalese people and Indian people along with their diaspora.
The Sun, considered as the god of energy and of the life-force, is worshiped during the Chhath festival to promote well-being, prosperity and progress. In Hinduism, Sun worship is believed to help cure a variety of diseases, including leprosy, and helps ensure the longevity and prosperity of family members, friends, and elders.
The rituals of the festival are rigorous and are observed over a period of four days. They include holy bathing, fasting and abstaining from drinking water ( Vratta), standing in water for long periods of time, and offering prashad (prayer offerings) and arghya to the setting and rising sun.
Although the festival is observed most elaborately in Mithila Region of Nepal, Terai-Madhesh of Nepal, Indian states of Bihar, Jharkhand and eastern UP, it is also more prevalent in areas where migrants from those areas have a presence. It is celebrated in all Northern regions and major Northern urban centers in India bordering Nepal. The festival is celebrated in the regions including but not exclusive to the northeast region of India, Madhya Pradesh, Uttar Pradesh, Chhattisgarh, Chandigarh, Gujarat Bangalore, Mauritius, Fiji, South Africa, Trinidad and Tobago, Guyana, Suriname, Jamaica, other parts of the Caribbean, United States, United Kingdom, Republic of Ireland, Australia, New Zealand, Malaysia, Macau, Japan, and Indonesia.