Tulsi Vivah is the ceremonial marriage of the Tulsi plant (holi basil) to the Vishnu or his Avatar Krishna.  This ceremony can be performed any time between Prabodini Ekadashi-the eleventh lunar day of the bright fortnight of the Hindu month Kartik in the full moon of the month (Kartik Poonima) but usually it is performed on the eleventh or the twelfth lunar day.  The day varies from region to region.  The Tulsi wedding signifies the end of the monsoon and the beginning of the Hindu wedding season.

This is venerated as a goddess in Hinduism and sometimes considered a wife of God Vishnu.  She is often called as Vishnupuria, the beloved of Vishnu.  

According to Hindu scripture, the Tulsi was a woman named Vrinda(Brinda), a synonym of Tulsi.  She was married to the demon King Jalandhar.  Due to her piety and devotion to Vishnu, her husband became invincible.  Even god Shiva, the destroyer in the Hindu Trinity could not defeat Jalandhar.  So Shiva requested Vishnu-the preserver in the Trinity – to find a solution.  Vishni disguised himself as Jalandhar and tricked Vinda.  Her chastity destroyed, Jalandhar was killed by shiva.  Vinda cursed Vishnu to become black in colour and he would be separated from his wife.  Thus, he was transformed into the black Shaligram stone and in his Rama avatar, his wife Sita was kidnapped by a demon-king and thus separated from him.  Vinda then driwned herself in the ocean.  Eventually Jalandar was killed by Shiva.  The Gods or Vishnu transferred her sould to a plant, henceforth which was called as Tulsi.  As a per a blessing by Vishnu to marry Vinda in her next birth, Vishnu in form of Shaligram – married tulsi on Prabodhini Ekadashi.  To commemorate this event, the ceremony of Tulsi ie performed.

Tulsi Vivah is celebrated on Kartik Dwadashi – A day after the Prabodhini Ekadash. The winter wedding season in India is inaugurated with Lord Krishna’s marriage with the sacred Tulsi plant, a symbol of Rukmini’s devotion and love for him.

Why is it considered holy? There are several traditions about this belief. But one that is relevant today and is touching to the heart concerns Lord Krishna’s love story with his consort Rukmini! It is said that when Krishna’s ‘work and duties’ on this earth were done, he was invited by all the gods to return to Devlok, the abode of divine beings. When he was gone, both his wives Satyabhama and Rukmini (Lakshmi) missed him and requested Muni Narada to bring him back! Narada said that Krishna would return only if either of them could outweigh him and offer something heavier than him to the gods. Satyabhama, arrogant and proud of her looks and riches, agreed instantly. Krishna was brought to the earth and asked to sit in one pan of a specially set up weighing scale (Tula) and Satyabhama quickly began to pile the other pan with her wonderful gems and gold and silverware. As she put more and more jewels, the pan in which Krishna was seated weighed heavier and heavier. The arrogant Satyabhama gave up in the end and retired to her palace. 

Rukmini – an incarnation of Lakshmi – came forward, plucked a sprig from the Tulsi plant growing nearby and put it in the pan emptied by Satyabhama, saying, “With all my love and devotion, I weigh this Tulsi sprig against you.” In an instant, the pan dropped, showing that the Tulsi leaves were heavier than Krishna! 

Since then, Tulsi leaves are a symbol of love and devotion and are loved by Krishna or Vishnu (Vithala) and offered to him in every ritual.