How the date is fixed
Maha Navami falls on the ninth day of the bright half of the lunar month Ashwina (Shukla Navami). The date is the day on which that tithi is running at sunrise (the usual rule, called udaya tithi).
About
Maha Navami is the ninth tithi of the bright half of Ashwina, in September or October, the closing day of Navratri. The tithi at sunrise is taken, and the Navami homa is completed before Dashami begins. In the south it is Ayudha Puja, when tools, instruments and vehicles are cleaned and worshipped, and in Tamil Nadu and Kerala it is also Saraswati Puja, with books set aside for the day. In Bengal it is the last full day of the Durga Puja before the immersion.
How it is observed
The Navami homa and kanya puja, Ayudha Puja of tools and vehicles in the south, and the farewell rites of Durga in Bengal.