When some one offers Lord Damodara a lamp on the full moon day of the month of Kartika, Lord Krishna, finding that He does not have sufficient money to repay that gift, gives Himself in exchange for that lamp.

This month is particularly dear to Lord Krishna, because it is the month of Srimati Radharani, the presiding deity of the month. Damodara is famous as bhakta-vatsala and this month He rewards even a tiny effort with tremendous results, actually thousand times more!!!

According to Skanda Purana, No sins exist anywhere in the three worlds that will not be purified by offering a lamp to Lord Keshava during Kartika.