Overview
Amavasya (Sanskrit: Amāvásyā) is the lunar phase of the new moon in the Hindu calendar. It marks the night when the moon is not visible, and is observed as a significant day in traditional Hindu timekeeping and ritual practice.
In the amanta tradition of the Hindu lunar calendar, a calendar month concludes on amavasya, with the following month commencing the next day. This convention is followed in several regions of India, while other regions observe the purnimanta tradition, in which the month ends on the full moon. The two systems differ in how months are demarcated, although the lunar phases themselves remain common across traditions.
A range of Hindu beliefs and rituals are associated with amavasya. The day is considered suitable for observances dedicated to ancestors and for various forms of devotional practice, and is regarded as spiritually significant within many Hindu communities.
Several Hindu festivals fall on the amavasya of particular months. Among the most widely observed is Diwali, the festival of lights, which is celebrated on the amavasya falling within a specific month of the Hindu calendar. Other festivals across different regions of India are likewise timed to coincide with the new moon day of various months.
References
Adapted from the English Wikipedia article on Amavasya.