Mahanavami falls on the ninth day of the Navratri festival and the third and final day of Durga Puja. On this occasion, Ma Durga is worshipped as Mahisasuramardini, which means annihilator of the Buffalo Demon. As per belief, the goddess killed Mahishasura after a long drawn out battle, a day after. Therefore, Mahanavani is dedicated to Goddess Durga by giving her a holy bath. She is also honoured with sixteen types of offerings.
The puja that is performed on this day is considered equal to worshipping the Goddess on all nine days of Navratri. On Mahanavami, the Siddhidatri form of the goddess is worshipped. Also, the girls who have not attained puberty are also worshipped as Ma Durga. Households invite them in their homes and are served with puri, chana, kheer and halwa and gifts. Some devotees also observe fast on this day.
On this day, the goddess is offered sugarcane stalks. Navami Homa is also performed after the end of the puja. Also, a Maha Arti is organised .
After the main bhog is offered to Ma Durga, devotees have it as part of the prasad. This practice is is done in several parts of the country.
Here are the Puja timings, according to drikpanchang.com:
Navami Tithi start = 9:36 PM on September 28
Navami Tithi Ends = 11: 49 PM on September 29