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 Dvadasi day, Vishnu woke up (Utthana means getting up) to perform His duty. It was on this day that the lord married Tulsi (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. The tulsi plant is watered and decorated with rangoli, haldi (turmeric), kumkum, flowers and gooseberry branches (nellikaai) and arathi, Lamps (jyothi) are lit around the vrindavan. An idol of Lord Krishna is traditionally kept near the plant and worshipped with flowers, milk and sweets like payasam made of beaten rice (avalakki), jaggery and coconut burfi. Prayers are offered for wealth, peace and prosperity and bhajans on Krishna and Tulsi are sung. Bursting of crackers is also common on this day.
The belief is that by worshiping Krishna and Tulsi, the head of the family attains moksha (exemption from further transmigration) and is blessed with material and spiritual benefits.