Lord Vishnu, after a long war, killed demon Shankasura and went to sleep in his ocean of milk on Ashadha Sukla Ekadasi day. While the Protector was asleep, the world and humans were vulnerable to destruction by evil powers. On Kartika Sukla Dwadasi day, Vishnu woke up (Utthana means getting up) to perform His duty. It was on this day that the lord married Tulasi (born by churning of the ocean of milk).
To celebrate the day, Hindu families keep a basil platform (vrindavan) on the right side of the house.