Four- day long Chhath festival began today with the worshipers- parvattin( from Sanskrit parb, meaning festival and attain meaning the one who practice it)- 
indulging in rituals of Nahai- Khai( taking bath and eating self-cooked atap rice, split chickpeas and pumpkin).

On the second, third and fourth day, parvattin will perform Khadna( cook Kheer-rice pudding), offering prayers to the setting Sun by putting half body inside waterbody and doing the same to the rising Sun god respectively.

Historically, the Chhathi Maiya is worshipped on the Chhath festival, which is mentioned in the Brahma Vaivarta Purana, according to Wikipedia.

It is said that the Chhath Puja was started in the holy city of Varanasi by Gahadavala dynasty. In Munger region, the festival is known for its association with Sita Manpatthar (Sita Charan; lit. Sita's footsteps). 

Sitacharan temple, situated on a boulder in the middle of the Ganges in Munger, is the main center of public faith regarding Chhath festival. It is said that goddess Sita performed Chhath festival in Munger. It was only after this event that Chhath festival started. 

That is why Chhath Mahaparva is celebrated with great pomp in Munger and Begusarai. According to some other legend, King Priyavrat, son of First Manu Swayambhu, was very sad because he had no children. Maharishi Kashyap asked him to do a yajna. 

According to Maharishis orders, he performed a yajna for a son. After this, Queen Malini gave birth to a son, but unfortunately the baby was born dead. The king and his family were very sad because of this. 

Then Mata Shashthi revealed herself in the sky. When the king prayed to her, she spoke, saying: "I am Chhathi Maiya the Sixth form of Devi Parvati. I protect all the children of the world and give the blessings of children to all childless parents." 

After this, the Goddess blessed the lifeless child with her hands, so that he came to life. The king was very thankful for the grace of the Goddess and he worshipped the goddess Shashthi Devi. It is believed that after this puja, this festival became a worldwide celebration.

 Chhath has been mentioned in both the major Indian epics. In Ramayana, when Rama and Sita returned Ayodhya, people celebrated Deepawali, and on its sixth day Ramrajya (lit. Kingdom of Rama) was established. 

On this day Rama and Sita kept fast and Surya Shashthi/Chhath Puja was performed by Sita. Hence, she was blessed with Lava and Kush as their sons. 

While in the Mahabharata, Chhath Puja was performed by Kunti after they escaped from Lakshagriha.

It is also said that Karna, the son of Surya and Kunti, was conceived after Kunti performed Chhath puja. Draupadi is also said to perform the Puja for Pandavas to win the Kurukshetra War.

The state governments of Jharkhand and Bihar keep offices closed for the festival. The Ranchi Municipal Corporation has cleaned all water bodies in Ranchi for the Parvattins of Chhath and their family members.