Tulsi Vivah is celebrated on Kartik Sud 15. Tulsi is the plant Ocimum Sanctum (a sacred venerated plant) and Vivah means marriage. Literally it is the celebration of Tulsi’s marriage.
Tulsi in her former birth was Vrinda, the faithful wife of the demon Jalandhar. She was exceptionally beautiful, maintained her dharma and was always the perfect wife to her husband. Jalandhar was so named because of being born in water (jala). He initially ruled over the oceans as he was born from the sweat which Shiva had thrown from his brow into the ocean when he perspired after being insulted by Lord Indra. This caused him to demand the fourteen treasures churned out from the ocean by Indra as part of his heritage. As was Indra’s nature he did not oblige. To avenge Indra, Jalandhar worshipped Brahma and underwent extraordinary penance. In delight Brahma granted him the boon that he would remain free from death as long as his wife remained virtuous and chaste.
Vrinda was renowned for her faithfulness and chastity. Jalandhar began to conquer all. He became extremely egocentric, thinking he was unbeatable, because of the strength of his wife’s devotion to him. So much so he waged war on Indra and Swarga [home of all the dev-gods]. The Gods fled to the protection of Shiva and Vishnu, but this was to no avail. Jalandhar had become invincible in the three worlds because of the devotion of his wife Vrinda. Vishnu and Shiva soon realised that the only way to defeat Jalandhar was to break the loyalty of Vrinda.
They deceived Vrinda by telling her that her husband had been killed in battle. They produced Jalandhar’s severed head and body. She thought her husband was dead and went into grief. In answer to her prayers bringing the severed body back to life Vishnu appeared before Vrinda as Jalandhar. Vrinda was deceived into thinking that it was her resuscitated husband, so she embraced him. Realising her mistake; she lost her virtue by touching another man: after which Jalandhar died in battle.
Upon discovering the identity and the horrible trick he had committed Vrinda cursed Vishnu, stating that he would become a black stone, Shaligram - the ammonite stone found mainly in the river Gandaki. In turn she too was cursed into becoming the Tulsi plant, her hair became the plant Tulsi used in the pooja to Gods; as a symbol of devotion. Vishnu won over by Vrinda’s unimpeachable chastity vowed to marry her every year, from therein. This has been immortalised by devotees performing their marriage annually on this holy day.
In the Hindu dharma, all Vaishanavas celebrate this day by re-enacting this marriage of Lord Vishnu and Tulsibai. It is celebrated with great pomp and splendour as this is the marriage of our Lord. Giving a daughter in marriage is considered to be an extremely meritorious act in the Hindu dharma, it is said one who performs this marriage does so considering Tulsi to be one’s own daughter, thereby gaining the credit of Kanyadan.