On this day Jahnu Muni swallowed and released the Ganga, who was on her way to deliver the ancestors of King Bhagiratha – this is one reason why the Ganges is also called the Bhagirathi.
On this day, the Ganga is worshipped, and many people make offerings to the forefathers, and bathe in the Ganga. On this day, if possible, one should worship the Ganges and bathe in her waters.
From Bhaktivinode Thakura’s Sri Navadvipa Mahatmya Chapter 13: Description of Vidyanagara and Jahnudvipa
As Nityananda