There are many stories that mention the belief that Goddess Lakshmi roams on the earth on the night of Sharad Purnima, a day which is also known as Kojagari Purnima. Many people make kheer or payasam and keep it exposed to moonlight before having it.
Is there any scriptural basis for the origin of this particular celebration and the rituals followed in it?
Yes there is. Chapter 124 (pg 30) of Volume 4 of Narada Purana mentions various vratas performed on Full Moon days. One such is Kojagiri Purnima.
Goddess Lakshmi is worshiped on this day. Lamps are lit and milk puddings are kept in the moonlight which are then offered as prasada to the deity.
A wake should be held at night enlivened with dance and songs. It is said Goddess Lakshmi visits the earth on this day and whoever she finds awake, worshiping her, she bestows with wealth.
47-48. The holy rite named Kojágara is to be observed on the full moon day in the month of Asvina.
The devotee takes the holy dip in accordance with the injunctions. He observes fast and subdues his sense-organs. A gold image of goddess Laksmi is placed in a pot made of copper or clay. It is covered with a cloth and worshipped with different kinds of Upacāras.
49.In the evening when the moon has risen, he should light lamps made of gold, silver or clay after filling them with ghee.
50-51. They should be a hundred thousands, fifty thousand, ten thousand, one thousand or hundred in number. Milk puddings are prepared in a huge quantity with plenty of sugar and ghee. They shall be kept in many vessels in the bright moonlight.
When the night has advanced by a full Yama (three hours), the devotee shall offer milk puddings as Naivedya to Lakşmī.
52.With great devotion he should then feed the Brahmanas with the milk puddings. All of them then keep awake for the whole of the night with songs and auspicious instrumental music and dances.
53-54. The next day before dawn, the devotee takes his bath and dedicates the gold image to the preceptor.
(It is said) that on this night goddess Mahalaksmi with the gestures of granting boon and fearlessness in her lotus-like hands moves over the Earth saying- "ko jagarti" (who is keeping awake) on this Earth? I shall give wealth to that person who keeps awake and worships me.'
55.This Vrata should be performed every year. It propitiates Laksmi. It yields prosperity in this world and bliss in the other world after death.
