The day when the sage Jahnu released the River Ganges after swallowing her. On this day, if possible, one should worship the Ganges and bathe in her waters.
From Bhaktivinoda Thakura’s Sri Navadvipa Mahatmya:
Chapter 13: Description of Vidyanagara and Jahnudvipa As Nityananda spoke, they went to Jannagara where they saw the enchanting hermitage of Jahnu Muni. Nityananda Prahhu explained, “This enchanting place named Jahnudvipa is known as Bhadravana. At this place, Jahnu Muni underwent penances and received darsana of the golden form of Gauranga. “Jahnu Muni was sitting here chanting his gayatri mantra, when his acamana cup fell into the Bhagirathi (Ganges) and was swept away by the currents. He opened his mouth and drank all the water in one gulp. King Bhagiratha thought, ‘Where has the Ganges gone?’ He concluded that Jahnu Muni had drunk the Ganges. The king was overwhelmed with anxiety and worshipped the muni for several days. The muni then released the Ganges from his body. Because of this incident the Ganges is known as Jahnavi, the daughter of Jahnu, throughout the world.”