Maha Saptami will be celebrated on Sunday, 5th October this year. It marks the seventh day of Durga Puja celebrations.
Durga Puja is a Hindu festival celebrated in South Asia that celebrates the worship of the ten-armed goddess Durga. Durga Puja celebrates her victory over the evil buffalo demon Mahishasura who threatened the entire world and was perceived to be invincible. To destroy the demon, Durga arose from the collective energies of all the gods. Each of her ten arms wielded the weapons of each god.
Thus, on the seventh day (saptami) of Durga Puja, the goddess started her battle against Mahishasura which ended with his death on Vijaya Dashami (the 10th day).