A large number of Muslims not only help to clean the banks of rivers and ponds where the devout gather, and donate money for the festival, some -- women in particular -- observe fast on account of Chhath.
"We have been celebrating Chhath for 34 years and will celebrate it till we breath our last," said Manjoor, in his 50's, from Lala Tola of Sitab Diara, the village of socialist leader Jayaprakash Narayan.
Manjoor, a drum beater, goes to river banks with his 'dug-dugi' during the festival.
Nazma, from Mohanpur Haat in Vaishali district, takes part in rituals -- like millions of Hindu women.
"I am not alone. There are dozens of Muslim women like me in nearby villages who celebrate Chhath," said Nazma, in her late 40s.
Muslim women in parts of Bihar have been celebrating Chhath for years, ignoring criticism from hardliners in their community.
It is a matter of faith, say the women, who pray to the sun "like our Hindu counterparts".
Ladli Begum of a village in Samastipur district has celebrated Chhath for eight years.
"It has brought good luck to my family. I am happy to observe fast and offer prayers to the sun," she said.
Ladli got attracted after her 10-year-old son fell ill and his condition became serious.
"An elderly neighbour suggested I perform Chhath. It worked like a miracle. My son's health improved and gradually he recovered fully," she said.
Mallika Bano, a resident of Shukla Road in Muzaffarpur, has observed Chhath for 25 long years -- and for good reason.
"My wish to have a son was fulfilled after I performed Chhath (years earlier)," she said.
Hindus see in these Muslims a show of communal harmony.
"It is a rare show of brotherhood as it promotes tolerance and peace," said Kanchan Bala, a woman activist.
The four-day Chhath festival comes after Diwali. It began Monday when devotees across the state bathed in a ritual called 'Nahai Khai' followed by Kharna Tuesday night.
Offerings of wheat, milk, sugarcane, bananas and coconuts are made to the sun god. The festival will conclude Thursday morning.
Hasan Imam, in his late 40s, a theatre activist from Begusarai district, said his family has performed Chhath for decades.
"There are several Muslim families in Begusarai and the neighbouring districts of Vaishali, Samastipur, Khagaria and Muzaffarpur who celebrate the festival," he said.
In Patna, Gaya, Bhagalpur, Muzaffarpur and Darbhanga, many Muslims have volunteered to clean up the banks of rivers, ponds and other water bodies and streets for the festival.
Some Muslims distributed fruits and new clothes to impoverished Hindu families on the occasion.