jalasya culukena va
vikrinite svam atmanam
[Sri Krishna – affectionate toward His devotees, is partial to a devotee who offers Him merely a tulasi leaf and a palmful of water]
Uttana Dwadashi (12th day in the Waxing phase of moon) in the month of Karthika is Celebrated as Tulsi Puja (or Tulsi Vivaha with Vishnu). It is also called Ksheerabdi Dwadashi Vrata. It is the day where the Chaturmasa vrata ends (Chaturmasa vrata starts on Ashada Ekadashi – Pandarapur Ekadashi)
The story behind this Vrata according to Aparna (my better half):
Tulsi known for her utmost devotion, piety and unflinching loyalty to her husband, Shankhachuda makes him extremely powerful. Shankhachuda falls into evil ways and starts harassing the world but was invincible due to Tulsi austerities. He has a boon that he can be killed only if chastity (Pativratata) of Tulsi is violated. Lord Vishnu appears in the form of Shankachuda and Tulsi mistakes her for her husband and serves him as loyally as always. Tulsi’s chastity and loyalty thus gets diluted making Shankhachuda weak and gets killed. Tulsi realized her folly and immolates herself on a pyre. Despite the devotion towards Lord Vishnu she was distressed that Lord was stone-hearted to trick her and curses Him that he turns into a black stone – Lord Vishnu thus turns into Saligrama stone. She also curses that Lord Vishnu suffers separation from his wife (Curse gets effected in Ramayana where Lord Rama and Sita are separated).
Puja at home (Wife on the left and Son on the right)
Lord Vishnu however blesses tulsi to take the form a Tulsi plant and insists that she forms an essential part of worshipping Him. He takes the form of Gooseberry plant and marries Tulsi. It is believed that Tulsi was born as Rukmini and marries Lord Vishnu’s incarnation – Krishna on the 12th Day (Dwadashi) on the shukla paksha (waxing phase of moon) in the month of Karthika. Tulsi puja is celebration of this marriage.
It is also believed that Lord Krishna who goes to sleep on Naraka Chatudashi day (first day of Deepavali) wakes up on the Twelfth day (hence called Uttana Dwadashi)
Worship include lighting of lamps with Ghee Wick held in a Gooseberry and Deepa-Daana (Donation of Lamp) in the evening. Gooseberry Stalk along with Tulsi plant to signify Lord Vishnu and Tulsi and the usual Puja samagris.