Tulsi Vivah is the ceremonial marriage of the Tulsi (holy basil) plant to the Hindu god Shaligram or Vishnu or to his avatar, Sri Krishna. The Tulsi wedding signifies the end of the monsoon and the beginning of the wedding season in Hinduism.
The ceremonial festival is performed anytime between Prabodhini Ekadashi (the eleventh or twelfth lunar day of the bright fortnight of the Hindu month of Kartik) and Kartik Poornima (the full moon of the month). The day varies regionally.
According to Hindu scripture, the Tulsi plant was a woman named “Vrinda” (Brinda; a synonym of Tulsi). She incarnated form of Goddess Lakshmi. She was married to the Asura king Jalandhar, who due to her piety and devotion to Vishnu, became invincible. Even Shiva could not defeat Jalandhar, so he requested Vishnu - the preserver in the Trinity - to find a solution. Vishnu disguised himself as Jalandhar and tricked Vrinda by touching her. She realized it was not her husband but Vishnu. This destroyed her chastity.
With her chastity destroyed, Jalandhar lost his power and was killed by Shiva. Vrinda cursed Vishnu to become Shaligram and to be separated from his wife, Lakshmi. This was later fulfilled when he was transformed into the black Shaligram stone (actually a fossil), and in his Rama avatar, was separated from his wife Sita, who was kidnapped by the asura king Ravana. Vrinda then drowned herself in the ocean, and the gods (or Vishnu himself) transferred her soul to a plant, which was henceforth called Tulsi.
As per a blessing by Vishnu to marry Vrinda in her next birth, Vishnu – in form of Shaligram - married Tulsi on Prabodhini Ekadashi. To commemorate this event, the ceremony of Tulsi Vivah is performed.
Another minor legend narrates that Lakshmi slew a demon on this day and remained on earth as the Tulsi plant.