Tulsi is considered the most sacred plant in Vaishnava tradition and is kept in a raised planter (Tulsi Vrindavan or Tulsi chaura) in the courtyard or balcony of many Hindu homes. Her leaves are offered in the worship of Vishnu and Krishna, and the plant is honoured daily with water and a lamp. Tulsi Vivah is the yearly festival in which the family celebrates her as a bride and joins her in marriage with Vishnu.
The festival comes right after Prabodhini Ekadashi, when Vishnu is traditionally believed to wake from his four-month rest (Chaturmas). Since auspicious ceremonies such as weddings are traditionally avoided during Chaturmas, Tulsi Vivah is seen as the first wedding of the season, and in many communities family weddings are planned only after it has been performed.
For many households, especially those without a daughter, performing Tulsi Vivah and giving Tulsi away in marriage (kanyadaan) is a cherished act of devotion. It is traditionally believed to bless the family with harmony and happiness in married life. It is also a joyful family occasion, with children, neighbours and relatives gathering to see the little wedding in the courtyard.
The traditional story, told in the Puranas, is of Vrinda, a woman of great devotion to Vishnu, who was married to the powerful asura king Jalandhara. Vrinda's faithfulness to her husband was so strong that it was believed to protect him, and no god could defeat Jalandhara in battle while it lasted. To end his attacks on the gods, Vishnu took the form of Jalandhara and came to Vrinda, and her vow was broken. Jalandhara was then defeated by Shiva.
When Vrinda learned what had happened, she was deeply grieved and cursed Vishnu to become a stone, which is why he is worshipped as the Shaligram. She then gave up her life, and from the place where she left her body the Tulsi plant grew. Moved by her devotion, Vishnu blessed her that she would be worshipped in every home, that no worship of his would be complete without Tulsi leaves, and that he would marry her each year in the form of the Shaligram. Tulsi Vivah is celebrated as the fulfilment of that promise, and as an honour to Vrinda's devotion.