Diwali falls on the new moon (Amavasya) of Kartik, the darkest night of the lunar month, and is celebrated with rows of lamps, sweets and family gatherings. It marks Rama's return to Ayodhya and is associated with Lakshmi, the goddess of prosperity.
The new moon is the fifteenth tithi of the waning moon — written above as krishna paksha, tithi 15.