This day marks the marriage of Lord Vishnu (Shaligram) with Tulsi Devi.
Vrinda, the daughter of Kushdhvaj, married the son of the Ocean Demi-God, Jaalandhar. Vrinda was very beautiful and chaste wife (Pativrata). This made Jaalandhar very powerful. He started harassing pious being all over the world with his atrocities.
Once he saw Devi Parvati and looked at her with lustful eyes. He challenged Lord Shiva to engage in a fight with him. Shiva was unable to gain victory over him due to Vrinda's protective power over Jaalandhar. Parvati pleaded to Lord Vishnu to put an end to Jaalandhar and the only way to resolve this was to break Vrinda's chastity.
Lord Vishnu took the form of Jaalandhar and stayed with her whilst the original was engaged in a fierce non-ending battle with Shiva. Thus, the vow of fidelity was unwittingly broken by Vrinda and Lord Shiva was able to annihilate the demon.
When Vrinda came to know about this, she cursed Lord Vishnu to become a black stone, which came to be known as Shaaligram. Lord Vishnu too cursed her to become a tree and hence Vrinda came to be known as Tulsi (Holy Basil).
Lord Vishnu gave Tulsi a boon that every year of Kartik Sud Poonam will be celebrated as their marriage ceremony which is known as Tulsi Vivah.