According to the mythology this festival is celebrated because tulsi plant which is said be representing the incarnation form of Mahalaxmi (Godess), also incarnation of Vishnu's wife (The God), she is also named as Vrinda (Brinda), a synonym of Tulsi. By some mistake she got married to demon king named Jalandhar. Due to her devoted prayer and dedication towards Lord Vishnu, he became very powerful and might that even lord Shiva could not destroy him (the destroyer of trinity) so he asked Lord Vishnu (the preserver of trinity) to find the alternative of this huge problem. Lord Vishnu masked himself as Jalandhar and exploited Vrinda. This destroyed her chastity and which gave a great opportunity to Lord Shiva to destroy Jalandhar. On this grief Vrinda cursed Lord Vishnu to turn into black colour and then separated his body from his soul, to take her revenge she burnt the body of Lord Vishnu into the funeral of her husband and even immolated herself into the fire due to the shame of being impure. Lord Vishnu after that transferred her pious soul into the tulsi plant. By the blessings of Lord Vishnu he promised to marry Vrinda in his and her next birth. Due to this auspicious blessing given by Lord Vishnu/Krishna in his next incarnation named Shaligram he married Tulsi on Prabodhini Ekadashi, in order to compensate the awful happening. To celebrate this marriage this Tulsi Vivah is celebrated all over India, mainly in Maharasthra.
This marriage (vivah) depicts the actual marriage of Hindu culture, where the bride is considered to be as Tulsi and groom is Shaligram (incarnation of Lord Vishnu). Every pujas or prayer of Lord Vishnu cannot be completed by Tulsi leave, when the parent give their daughter in the hand of groom it is given by taking one Tulsi leave in his hand, depicting meritorious to the couple. The bride is offered with Mangalsutra which is the main ornament of a women, groom is given with scared sandalwood paste and an auspicious thread. This is all rituals to be performed for the progress and prosperity of the married couple.