Chhath is a four-day observance to Surya and to Chhathi Maiya, kept most strictly in Bihar, Jharkhand, eastern Uttar Pradesh and the Nepali Terai. Its central day is the Shashthi of Shukla Paksha in Kartika. It is among the very few rites in which the setting sun is worshipped before the rising one — arghya is offered at dusk on the third day and at dawn on the fourth.

That order is the meaning of the festival. The sun is thanked as it declines, before it is greeted as it returns. Nothing is asked of it at the moment of its strength.

Chhath has no priest. The vratti — most often a woman, though men keep it too — performs every part herself, standing waist-deep in water. Its austerity is severe: a nirjala fast of roughly thirty-six hours, without even water, spanning the two arghyas.

Chhath is offered to Surya, the sun, and to Chhathi Maiya, the goddess of the sixth day who guards children and the home. Karna, the sun's own son, is remembered standing waist-deep in water at dawn giving arghya — the posture of the vratis to this day. The Mahabharata tells that Draupadi and the Pandavas kept Chhath on the sage Dhaumya's counsel and regained their kingdom; another telling has Sita keeping it at Munger after the return to Ayodhya. It is worship without a priest, an idol or a temple — only the sun, the water and the vow.

Chhath falls in Kartik, after the monsoon, when the low-angled morning and evening sun is gentlest and its light richest in the spectrum the body needs. The vratis stand in a river or pond and take the sun directly — arghya to the setting sun on the third day and to the rising sun on the fourth. It is one of the very few observances anywhere to honour the setting sun as fully as the rising: a thanks to the source of all life at both its going and its coming.

That the setting sun is honoured first is the heart of Chhath: decline is met with the same gratitude as ascent, endings with the same reverence as beginnings. The vrat is austere to the point of severity — a thirty-six-hour fast without water, kept in public at the water's edge — and it belongs almost entirely to women, offered not for themselves but for the health and continuance of the family.

When the Pandavas lost their kingdom at dice, Draupadi was counselled to worship Surya. She kept the vrat, offered arghya at dusk and at dawn, and what had been lost was in time restored. Karna, Surya's own son, is said to have stood waist-deep in water each morning offering water back to his father, and to have given away whatever was asked of him for as long as he stood there.

Chhathi Maiya is invoked alongside the sun as Usha and Pratyusha, the first and the last light. She is called the sixth form of Prakriti and the protector of children, which is why the vrat is so often kept for a child's life. The absence of any priest is deliberate: what passes at the ghat passes directly between the vratti, the water and the sun.