Vaikuntha Chaturdashi is a holy day in the Hindu calendar that is observed one day before Kartik Purnima. The Shukla Paksha Chaturdashi during Kartik month is considered sacred for the devotees of Lord Vishnu as well as Lord Shiva because both deities are worshipped together on the same day. Otherwise, it is very rare that Lord Shiva and Lord Vishnu are worshipped together on the same day.
Most of the temples in Varanasi observe Vaikuntha Chaturdashi and it falls one day before another significant ritual of Dev Diwali. Apart from Varanasi, Vaikuntha Chaturdashi is also observed in Rishikesh, Gaya and many cities in Maharashtra.
According to Shiva Purana, on the auspicious day of Kartik Chaturdashi, Lord Vishnu went to Varanasi to worship Lord Shiva. Lord Vishnu pledged to worship Lord Shiva with a thousand lotuses. While offering lotus flowers, Lord Vishnu found that the thousandth lotus was missing. To fulfill and complete His Puja Lord Vishnu, whose eyes are compared to lotuses, plucked one of His eyes and offered to Lord Shiva in place of the missing thousandth lotus flower. This devotion of Lord Vishnu pleased Lord Shiva so much that He not only restored the plucked eye of Lord Vishnu but also rewarded Lord Vishnu with the gift of Sudarshana Chakra which became one of the most powerful and sacred weapons of Lord Vishnu.
On Vaikuntha Chaturdashi, Lord Vishnu is worshipped during Nishitha which is midnight as per the Hindu division of the day. Devotees offer Lord Vishnu a thousand lotuses while reciting the Vishnu Sahasranama, the thousand names of Lord Vishnu.
Although both Lord Vishnu and Lord Shiva are worshipped on the day of Vaikuntha Chaturdashi, the devotees perform Puja at two different timings of the day. The devotees of Lord Vishnu prefer Nishitha which is Hindu midnight while the devotees of Lord Shiva prefer Arunodaya which is Hindu dawn for the Puja. For the devotees of Shiva, the early morning bath during Arunodaya at Manikarnika Ghat in Varanasi is very significant and this holy dip is known as Manikarnika Snan on Kartik Chaturdashi
This is the only day when Lord Vishnu is given a special place of honor in the sanctum of Kashi Vishwanath Temple, a prominent Lord Shiva temple in Varanasi. It is believed that the Vishwanath temple becomes as pious as Vaikuntha on the very same day. Both deities are ritually worshipped as though they are worshipping each other. Lord Vishnu offers Tulasi leaves to Shiva and Lord Shiva, in turn, offers Bael leaves to Lord Vishnu.