Navaratri is a Hindu festival that spans nine nights (and ten days) and is celebrated every year in the autumn. It is observed for different reasons and celebrated differently in various parts of the Indian cultural sphere.
Theoretically, there are four seasonal Navaratri. However, in practice, it is the post-monsoon autumn festival called Sharada Navaratri that is the most observed in the honor of the divine feminine Devi (Durga). The festival is celebrated in the bright half of the Hindu calendar month Ashvin, which typically falls in the Gregorian months of September and October.
In the eastern and northeastern states of India, the Durga Puja is synonymous with Navaratri, wherein goddess Durga battles and emerges victorious over the buffalo demon to help restore Dharma. In the northern and western states, the festival is synonymous with "Rama Lila" and Dussehra that celebrates the battle and victory of god Rama over the demon king Ravana. In southern states, the victory of different goddesses, of Rama or Saraswati is celebrated. In all cases, the common theme is the battle and victory of Good over Evil based on a regionally famous epic or legend such as the Ramayana or the Devi Mahatmya.