1. Chhath puja is mainly observed in Bihar and Nepal. Bihar has a number of Sun temples, flanked by a surajkund or sacred pool of the Sun. 2. It is a ritual bathing festival that follows a period of abstinence and ritual segregation of the worshiper (parvaitin) from the main household for four days. The parvaitin observes ritual purity, and sleeps on the floor on a single blanket. 3. This is the only festival which does not involve any priest. 4. The devotees offer their prayers to the setting sun, and then the rising sun. 5. It is the most glorious form of Sun worship. 6. The main worshippers (parvaitin) are women. 7. The parvaitin pray for the well-being of their family, for prosperity and for offspring. 8. The prasad offerings include sweets (Thekua) and fruit offered in small bamboo winnows. 9. The food is strictly vegetarian and it is cooked without salt, onions or garlic. 10. Day 1: Nahay khay (bathe and eat): The parvaitin take a dip in river. The house is cleaned. Only one meal is eaten and contains �kaddu-bhat� or channa dal, and arwa chawal (rice). 11. Day 2: Kharna (the day before Chhath): On Panchami, the parvaitins observe a fast till evening a little after sunset. Just after the worship of earth, the offerings of Rasiao-kheer (rice delicacy), puris (puffs of wheat flour) and bananas, are distributed. From time onwards, for the next 36 hours, the parvaitin goes on a fast without water. 12. Day 3: Chhath: Sanjhiya Arghya (evening offerings): The day is spent preparing the Prasad (offerings) at home. Offerings (Argh) are made in the evening to the setting sun. In the night a colorful event of Kosi is held. Here, lighted earthen lamps are kept under a canopy of five sugarcane sticks. The five sticks signify the human body made of five elements. 13. Day 4: Parna (day after Chhath): Bihaniya Aragh (next morning offerings): On this final day the parvaitin with family go to the riverbank before sunrise and offer (Aragh) to the rising sun. The festival ends with the breaking of the fast. Science behind the pooja 1.Sun worship in Kartik month is related to absorption of vitamin D. 2. Vitamin D comes from UVB rays. 3. These rays are predominant at sun set and sun rise. 4. Vitamin D deficiency today is running like an epidemic in the society. 5. Vitamin D is sued for absorbing calcium from the food. 6. All the food items used in this pooja are high in calcium. 7. Kartik is also high fertility months. Vitamin D deficiency is linked to infertility. 8. Natural calcium is better absorbed in fasting state. 9. Patients with compromised kidneys should not do this fast. 10. Fast is never broken with feast. This fast is usually broken with ginger and jaggery. (Disclaimer: The views expressed in this write up are my own).