Sharad Poornima or Kojagar Poornima is celebtated on the full Moon Day of Ashvin Masa of hindu calender. It is September - October end of monsoon and starting of winters. It is also called kaumudi celebration or celebration in moonlight.
It said that The Goddess Lakshami will spread her complaisance on the person who awake and vigil this night and worshiped her. This satisfies Goddess Lakshmi. Many people make kheer (a dish made from milk, rice flakes and sugar) on this night and put it the whole night in the moon light and eat it as dainty of lord. It is strongly believed that goddess Lakshami will surely praise him with luxuries for life long, who do worship to the goddess Lakshmi. It says that this night moon shower elixir with his cool rays.
An Interesting story is about Sharad Purnima, is that once all the gopied get umbrage with krishna that he don’t give time to them because of Raadha. So Lord Shri Krishna promises all the Gopis to do Maharas the whole night of this Purnima with the Gopis. Thus Shri Krishna forms the Maharasa on this Day. But after that he got message from lord Shiva that he want to meet him. Lord Krishna denied him with regret, but Lord Shiva come on the earth to meet Shri Krishna in a Gopi's disguise and get into the Maharasa. As in Maharas no man allowed except Krishna, Krishna recognized the Lord Shiva. Lord Shiva feels his mistake. Krishna smiled at Lord Shiva and accepts him as his Gopi.
Another story about Kojagar Purnima is once a business man has two daughters, observing fast of purnima. The elder girl observ fast for whole day, but the younger girl observe fast only for half a day. As result younger girl's all children died while birth. When she asked the reason from pandit, he replied that you were doing fast for half a day. That’s why this happen, so you have to fast for the whole day. Then she observed the fast for full day like her elder sister, but again his son died at the time of birth.
Then she put the dead body of his son on a seat and covers it with a cloth, and offers the seat to her elder sister. While seating, her skirt touches his boy and it started crying. The elder girl get surprised and scolded on her younger sister that if she gets seated on the boy it will get died. Then the younger girl admits that the boy was already died. But after touching you it get alive, it is the result of your saintly behavior. And order to the entire citizen to observe fast on every purnima in the city.