Tulsi plant ( Basil) is worshiped as a Goddess by Hindu. and considered the wife of Lord Vishnu.
Tulsi Vivah is the ceremonial marriage of he Tulsi Plant to Vishnu. It is usually performed on the eleventh on twelvth lunar day in the month of Kartik (around October).
According to Hinduism, Tulsi plant was a woman named Vrinda. She was married to the demon king Jalandhar. Due to her devotion to Lord Vishnu, her husband became invincible. Even Lord Shiva could not defeat Jalandhar. So Shiva requested Vishnu, the preserver of the Trinity to find a solution.
Vishnu disguised himself as Jalandhar and stayed with her. Later, Jalandhar was killed by Shiva. Vrinda cursed Vishnu to become black in color and said he would be separated from his wife. Thus he was transformed into the Black Shaligram stone and in his avatar as Rama,and his wife was taken away by Ravan and Rama was separated from his wife.
Vrinda burnt herself at the pyre of her husband . Vishnu transformed her soul into a plant Tulsi. Vishnu in the next birth married Tulsi. To commemorate this event , the cermony of Tulsi Vivah is conducted every year in temples and at homes of those desirous of Kanyadan.
The expense of the wedding is usually borne by a daughter-less couple who act as parents. My friend who has 2 sons, performed the Vivah last year at home. The giving away ( Kanyadan) of Tulsi to Vishnu is considered meritorious for the couple. The Bridal offerings to Tulsi are given to a Brahman priest.
The Tulsi plant decorated as the bride:
Shaligram Bhagwan being brought in his Palki:
Tulsi and Shaligram set for the marriage:
Tulsi and Shaligram after the Jaimala and Vivah: