(Click to enlarge photo of Radha-Kunda at midnight—Snana Dana Bahulastami)
After killing a bull-demon named Aristasura, Krishna approached Radharani to engage in loving affairs. However, She rebuked Him, stating He was impure because of committing the sin of killing a bull, the symbol of religion. She suggested that to counteract this sin He should bathe in all the sacred rivers.
Krishna, being eager to engage in loving affairs with Srimati Radharani, immediately shoved His lotus foot into the ground, making a large hole, and called all the sacred rivers to enter into it. They appeared before Him and formed Shyama-kunda (the bathing place of Shyama—one who is the color of a dark rain cloud).
After bathing, Krishna stated that He had become pure by bathing in Shyama-kunda, but that the gopis had become impure, having sided with a demon, even though he appeared in the form of a bull. Radharani, along with some of Her gopi friends, broke their bangles and dug a hole with the broken pieces. But there was no water, so Krishna laughed. The gopis then formed a line to the Manasi Ganga and started to fill the hole with water that was passed from one gopi to another. At that time, all the sacred rivers arrived in person and requested permission to enter into Radharani's kunda, which they did after receiving Radharani's blessing.
These two kundas manifested at midnight on Krishnastami—the eighth day of the dark moon—in the month of Kartika. Hundreds and thousands of people therefore bathe here at midnight on this day known as Bahulastami.
Lord Caitanya Discovers Radha-Kunda
"In Caitanya-caritāmṛta (CC Madhya) it is stated that when Śrī Caitanya Mahāprabhu first visited the area of Vrajabhūmi, He could not at first find the location of Rādhā-kuṇḍa. This means that Śrī Caitanya Mahāprabhu was actually searching for the exact location of Rādhā-kuṇḍa. Finally He found the holy spot, and there was a small pond there. He took His bath in that small pond and told His devotees that the actual Rādhā-kuṇḍa was situated there. Later the pond was excavated by Lord Caitanya's devotees, headed first by the six Gosvāmīs, such as Rūpa and Raghunātha dāsa. Presently there is a large lake known as Rādhā-kuṇḍa there. Śrīla Rūpa Gosvāmī has given much stress to Rādhā-kuṇḍa because of Śrī Caitanya Mahāprabhu's desire to find it. Who, then, would give up Rādhā-kuṇḍa and try to reside elsewhere? No person with transcendental intelligence would do so. The importance of Rādhā-kuṇḍa, however, cannot be realized by other Vaiṣṇava sampradāyas, nor can persons uninterested in the devotional service of Lord Caitanya Mahāprabhu understand the spiritual importance and divine nature of Rādhā-kuṇḍa. Thus Rādhā-kuṇḍa is mainly worshiped by the Gauḍīya Vaiṣṇavas, the followers of Lord Śrī Kṛṣṇa Caitanya Mahāprabhu."
(Nectar of Instruction 9, Purport)