“Of the many objects of favored delight and of all the lovable damsels of Vraja, Srimati Radharani is certainly the most cherished object of Krishna’s love. And, in every respect, Her divine Kunda is described by great sages as equally dear to Him. Indeed, Radha-kunda is rarely attained even by the great devotees; therefore, it is even more difficult for ordinary devotees to achieve. If one simply bathes once in these sacred waters, pure love for Krishna is awakened within the heart.”
— The Nectar of Instruction, Verse 11

Sri Radha-kunda manifested on the eighth day of the waning moon in the month of Kartik. This sacred day is observed by bathing in the holy waters at midnight — the very time when Radha-kunda appeared. On this night, thousands of devotees gather on the banks of Radha-kunda in Vrindavan to celebrate. They offer not only gifts like water, milk, fruits, sweets, garlands, lamps, and donations, but also their minds, bodies, and souls in loving devotion. United in prayer, they glorify Sri Radha-kunda, who is non-different from Srimati Radhika Herself.

In his purport to Srimad Bhagavatam (10.36.16), Srila Visvanatha Cakravarti Thakura beautifully describes the divine pastime of the appearance of Radha-kunda.

The appearance of Radha-kunda is also known as Bahulastami, named after the sacred pastime of the cow Bahula who took shelter of Lord Krishna. Near Krishna-kunda in Vrindavan, one can see the deities of a cow and her calf, a brahmana, a tiger, and Lord Krishna — commemorating this beautiful story.

Once, while the cow Bahula was peacefully grazing, a tiger suddenly attacked her. Bahula humbly requested the tiger to allow her to return to her calf, feed it, and bid farewell before surrendering herself. Moved by her sincerity, the tiger agreed. Bahula went to her master, a brahmana, and her calf, explaining what had happened. Out of compassion, both the brahmana and the calf offered to go in her place as the tiger’s meal. Despite their pleas, Bahula refused, but ultimately, all three — the cow, calf, and brahmana — went together to the tiger, ready to sacrifice themselves.

As the tiger pondered which to eat first, Lord Krishna, the protector of Dharma and all the residents of Vraja, appeared before them, holding His chakra. He blessed the tiger, assuring him of eternal fame if he spared the innocent cow and her kin. The tiger released Bahula, and all were overjoyed by Krishna’s merciful intervention.

Pleased with Bahula’s truthfulness and purity, Krishna named that forest Bahulavan and blessed her to become His consort. In her next life, Bahula indeed became one of Krishna’s wives. Thus, this sacred day came to be known as Bahulastami.

Bahulavan is considered by many Vaisnavas to be the most sacred of all forests, for within it lies the holiest place — Sri Radha-kunda. Srila Rupa Goswami, in Sri Upadesamrta, confirms the supreme position of Radha-kunda with the following verse:

“The holy place known as Mathura is spiritually superior to Vaikuntha because the Lord appeared there. Superior to Mathura is the transcendental forest of Vrindavan, the site of Krishna’s rasa-lila pastimes. Higher still is Govardhana Hill, raised by Krishna’s divine hand and witness to His loving exchanges. Yet, above all, the super-excellent Sri Radha-kunda stands supreme, overflowing with the ambrosial nectar of Sri Krishna’s prema.”

In Sri Navadvipa-dhama, Sri Radha-kunda has manifested Herself in two locations — on the islands of Rtudvipa and Antardvipa.

In Navadvipa-bhava-taranga, Srila Bhaktivinoda Thakura describes Lord Chaitanya Mahaprabhu’s mood as He visited Rtudvipa:

ritudvipam tato gatva
drishtva sobham vanasya ca
radha-kundadikam smritva
ruroda saci-nandanah

“Upon arriving in Rtudvipa and seeing the beauty of the forest, Saci-nandana (Sri Chaitanya) began to cry, remembering Radha-kunda and other sacred places.”

In Antardvipa, within the home of Chandrasekhar Acharya, Mahaprabhu’s maternal uncle, stands Sri Chaitanya Math — the headquarters of the Gaudiya Math established by Srila Bhaktisiddhanta Sarasvati Thakura. Following his guidance, disciples brought sacred waters from Radha-kunda and Syama-kunda, Giriraja silas from Govardhana Hill, and Tamala and Keli Kadamba trees from Vrindavan. Through his deep meditation and devotion, Srila Bhaktisiddhanta Sarasvati Thakura perceived these sacred places as being fully manifested within the Math.

All glories to the divine appearance of Sri Radha-kunda — the holiest of all holy places!

References:
Sri Upadesamrta, Srimad Bhagavatam 10.36.16 (purport), Navadvipa-dhama-mahatmya (Pramana-khanda 4.44), Nava Vraja Mahimamrta, Vol. 2 by Sivarama Swami, and Vedabase.