Sharad Purnima, also known as Kojaagari Purnima, is celebrated on a full moon day of the Hindu lunar month of Ashwin.
On Sharad Purnima, the moon and the Earth are very close to each other. The rays of the moon have some healing properties which is said to nourish the body and soul of an individual. That is why food kept in the moonlight on Sharad Purnima is considered equal to Amrit (the immortal drink) or elixir. Hence, people prepare kheer and keep it open under the moonlight throughout the night of Sharad Purnima. The next day it is consumed by all the members of the family as it becomes 'Amrit' according to local beliefs.
The practice of drinking cold milk and rice flakes during this fast has its origins in science. Sharad ritu brings in very hot days and cool nights. During this weather, ‘pitta' or acidity becomes predominant in our body. Consumption of milk & rice flakes is a good remedy for ‘pitta'. Thus, it is customary to consume cold milk and rice flakes on Sharad Purnima.