Mahalaya Amavasya is the day that marks the culmination of the Pitru Paksha period and the beginning of the Devi Paksha. Mahalaya Amavasya is significant because it is also called Sarma Pitru Amavasya Shradh. On this day, Shradh and Tarpan rituals are performed for those who breathed their last on Purnima Tithi, Chaturdashi Tithi and Amavasya Tithi. It also marks the onset of the Sharadiya Navratri season that is dedicated to Mae Durga, The devotees keep a fast for nine days and worship the nine forms of Mae Durga. On this day the Goddess descends on the earth from her heavenly abode or Kailash. The Mahalaya Amavasya is also called the Sarma Pitru Amavasya Shradh day. It is believed that the Goddess arrives with her children, Kartikeya, Ganesha and Goddesses Lakshmi and Saraswati on a vehicle of her choice. The vehicle could be a lion, boat, horse or elephant. The mode of transport changes every year and even when she departs. People begin preparations to welcome the Goddess on the day of Mahalaya Amavasya although the Durga Puja celebrations last for five days. People begin the preparations by listening to the Chandi Path by 4 am.
In West Bengal, Mahalaya Amavasya marks the beginning of Navratri festivity. It is believed that Goddess Durga was descended on the Earth on this day. On the day of Mahalaya, people pay obeisance to the soul of their father by performing tarpan in Ganga. This day is also ideal to conduct Shradh rituals for anyone who breathed their last on the Purnima Tithi, Chaturdashi Tithi or Amavasya Tithi.
It is also the day when Devipaksha begins.