Maha Navami is part of the Navaratri observance rooted in the Devi Mahatmya, with regional forms such as the Bengali Ayudha and Saraswati puja.
On the ninth day, the goddess took the form of Siddhidatri, bestowing spiritual powers, as she approached the final victory over the demon Mahishasura on the following day.
Devi Mahatmya; regional Navaratri tradition