“One day Nanda Maharaj and the cowherd men heard the glories of Ganga: Once Garuda was carrying a snake in his claws and was flying over Ganga, when accidentally the tail of the snake touched the waters of Ganga and the snake immediately transformed to a four handed form of the residents of Vaikuntha. Amazed, Garuda then placed that divine being on his back and took him to Vaikuntha.
Hearing this pastime, Nanda Maharaj and the others desired to go to the Ganges and take bath in her. But Krishna stopped them and explained that all the holy places resided in Vrindavan, and therefore there was no need to go out of Vrindavan. He took them to Manasi Ganga. At that point, Mother Ganga personally came out on her crocodile. When the Vrajavasis saw Mother Ganga they offered obeisances to her. Nanda Maharaj and the others then took their holy bath in Manasi Ganga. This incident which took place on the day of Diwali is still commemorated by offering lamps to Manasi Ganga on that day.
– Radhanath Swami