Each month of the lunar calendar comes with one Purnima or full moon night and one Amavasya or full dark night. The Amavasya heralds the arrival of the new month as the moon starts its waxing phase from the very next day again. The Amavasya thus also symbolises new beginnings and end of everything that went previously.
Rishi - Munies in ancient ages feared the black Amavasya night due to utter darkness. Due to the extreme black night, people took this day off and spent it as a day of worship and devotion. People offered prayers to Gods and paid homage, in many regions, to their descendents or ancestors, on this day.
Many important festivals fall on Amavasya night, most important of which is Diwali or Deepavali.