Celebrated from the sixth day (hence called 'Chhath') after Diwali, it is celebrated to pay obeisance to the Sun as cosmic energy.
The puja is observed mainly by the people from Bihar and eastern Uttar Pradesh or Poorvanchalis.
The national capital has a sizeable population of people from these two states, who observe it here on the banks of the Yamuna or ponds, lakes and canals.
Chhath, this year, is being celebrated from October 24- 27. The devotees offer 'arghya' to the setting sun and the rising sun in succession on the last two days of the puja.
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Today marks the ritual of 'Nahai-Khai' when devotees prepare traditional food after bathing.
The second day is called 'Kharna', during which devotees observe a day-long fast which ends with sunset. They then cook 'kheer' (pudding) and roti on earthen chullahs, which is then distributed as 'prasad'.
Vegetables, fruits and other natural products are offered to the Sun God on the last two days, and 'thekua' - a delicacy made out of fine flour, ghee and sugar -- is the main 'prasad', which is then distributed after the offerings.
Markets in the city are wearing colourful look, with big bunches of bananas and citrus fruits on sale as customers jostled to buy those in wholesale.
The government had yesterday said the third day of the festival will be a holiday. Last year too, it had declared a holiday on the occasion.
Various civic leaders today inspected ghats in their respective areas to take stock of the preparations. Delhi Development Minister Gopal Rai yesterday had visited Shyam Ghat at Jagatpur in the Wazirabad area for the same.
"This year, Chhath Puja is being observed at 565 ghats while last year it was organised at 268 ghats. The government had earlier constructed 50 permanent ghats for the celebrations," a senior official said.