It must be noted her that the period beginning from Ekadasi (after Amavasi) in Kartik month to Purnima (the full moon day) in Kartik Month is considered auspicious for Tulsi Vivah. Majority of the people perform it on the Ekadasi day or on Purnima day. The ritual is also performed on the day after Ekadasi in many regions. Any day which falls between Prabodini Ekadasi and Kartik Purnima is considered auspicious to perform the puja.
Tulsi Vivah is an age-old tradition and is mentioned in the Padma Purana and other ancient scriptures. Tulsi, or the Holy Basil, is the sacred plant of the Hindus and symbolizes purity and is famous for its medicinal properties.
Legend has it that a king named Jalandhar got a boon that he will remain immortal as long as his wife Vrinda is chaste (Pati Vrat). This made King Jalandhar arrogant and he declared war on other kings, demi gods and started tormenting innocent people. Finally, the task of annihilating King Jalandhar fell on Lord Vishnu.
Lord Vishnu in the guise of Jalandhar approached Vrinda and she mistook him for her husband. They lived like husband and wife. Soon Jalandhar was killed in a battle. This made Vrinda furious and she cursed Vishnu and made him turn to Shaligram.
Lord Vishnu did not let Vrinda suffer for no fault of her. She was turned into a Tulsi plant and gave her the boon that she will find a place in all houses and she will be worshiped. Tulsi Vivah is performed to establish this divine relationship between Lord Vishnu and Vrinda.
The festival of Tulsi Vivah is also the precursor to the annual marriage season in North India. Performing it is considered highly auspicious and good for couples. Popular belief is that performing the wedding ceremony of marrying Tulsi to Vishnu considering her as their daughter is equal to performing Kanyadan.