The moon goes around the earth once in every 29.5 days. The moon cannot glow on its own and it just reflects the sun rays falling on it. During its various positions around the earth, it is expose to varying amount of sun rays as the earth occurs in between. Therefore moon appears to grow and reduce in its size. When the earth completely blocks the moon surface from the sun, it is Amavasya and when the moon is completely expose to the sun, it is Purnima.
Usually, Hindu tradition believes that during Amavasya, the negative forces and evil powers reign over the earth. Amavasya is dedicate to performing the customary rituals for the deceased ancestors. No positive things or auspicious activities are started on Amavasya.
Some festivals however fall on Amavasya, Diwali being the major one. Diwali is infact celebrate as the festival of light to ward of the evil powers and establish the supremacy of divine energy.
In Sanskrit, “ama” means “together” and “vásya” means “to dwell” or “cohabit”. It also means “na” +”ma”+”asya” meaning to “na” = “No, “ma”=Moon, “Asya”=”There” in turn meaning to There is no Moon i.e., Moon is not visible.
Amavasya is consider a night when all activities related to black magic or evil spirits are more prominent. Some people even avoid stepping out of their home on this night. Religious people are not suppose to travel or work, and instead concentrate on the rites of Amavasyas, typically at home in the afternoon. Even today, traditional workers like masons do not work on Amavasya in India. However, they will work on Saturdays and Sundays. Even High Court judges of 18th century India used to observe Amavasya as a day off. It was the British Rule that brought the Christian Sunday-off principle to Indian industry. People even observe a strict fast on this day to keep all these evil spirits at bay and devotedly worship Lord Shiva.
An Amavasya falling on Mondays has a special significance. It is believe that a fast on this particular Amavasya would ward off widow-hood in women and ensure bearing of progeny. It is also believe that all desires could be fulfill if one fasts on this Amavasya.
Every month, the Amavasya day is consider auspicious for the worship of forefathers. The dark fortnight of Aswayuja (September–October) is observe as the Pitru Paksha (Mahalaya). Which is especially sacred for offering oblations to departed ancestors. The last day of this period, the dark moon day, called mahalaya Amavasya. It is consider the most important day in the year for performing obsequies and rites. The manes return to their abode on the evening of Deepavali. Due to the grace of the Yama, it has been ordained that offerings made during this period benefit all the departed souls, whether they are connected to you or not.