In Bengal, the Ganga Sagara festival (Mela) is held on Makara Sankranti (15th January this year) when the sun enters Capricorn. Sagara means the ocean, and the island is located where the river Ganga enters the ocean. A fair is held and people come from long distances to bathe here. After the bath, Goddess Ganga is worshipped. It is believed that Ganga entered the ocean at this point to wash the bones of the 60,000 sons of Sagara, in whose memory the island is held sacred. They were killed by a curse from sage Kapila.
Festivals connected with rivers are usually bathing festivals. River Ganga is worshipped as a mother as well as a Goddess, particularly by people of Uttar Pradesh, Bihar, and Bengal through which the river flows. On this day, if a devotee is unable to visit and bathe in the River Ganga, then Ganga jal (water) kept in most Hindu homes is used for purification. A bath in the river is said to purify the bather of all sins.
Originally, river Ganga flowed in the heavens. She was brought down to earth by the severe penances of sage Bhagiratha and that is why she is also called Bhagirathi. A number of demons were once harassing the hermits by disturbing them in their ascetic duties. During the day, they would be chased into the ocean. But at night, they would emerge from the ocean and start harassing the hermits again. The hermits appealed to Rishi Agastya. Agastya drank all the water from the ocean. Though this was done in good faith, it resulted in depriving the world of the water needed for sustenance and the earth became parched and dry. Bhagiratha brought this drought to an end.
The Agni Purana and Padma Purana state that the Ganga descended to the earth on Ganga Dussehra day and a bath in the holy river on this day is said to purify one of all sins. To die on the banks of the Ganga is considered most auspicious. If that is not possible, then the immersion of the ashes after cremation in the river Ganga is a must, as it then releases one from the cycle of birth and death.
It is understood that Kapila Muni first went towards the Himalayas and traced the course of the River Ganges, and He again came to the delta of the Ganges at the sea now known as the Bay of Bengal. The ocean gave Him residence at a place still known as Ganga-sagara, where the River Ganges meets the sea. That place is called Ganga-sagara-tirtha, and even today people gather there to offer respects to Kapila deva, the original author of the Sankhya system of philosophy.
– Srimad Bhagavatam, 3.33.34, Purport