Kārttika-śukla-dvādaśī (twelfth day in the bright fortnight of the month Kārttika [October/November]) is known as Utthānadvādaśī. It was on this day that Tulasī or Vṛndā was married to Hari or Viṣṇu. Hence this day is also called Tulasīvivāha day.
On this day ceremonial marriage of the Tulasī plant (hole basil) with Lord Hari is performed.
In some Hindu homes, images of Hari and Tulasī are kept from the navamī to ekādaśī (9th day to 11th day) and ceremonially married on the 12th.
See also TULASĪ.