The festival of Durga Puja starts with Mahalaya, the first phase of the waxing moon in Aswin. Thousands offer prayers to their ancestors at the city's river banks, a ritual called Tarpan. The inauguration of the Goddess idol starts on Mahashasthi. The main puja is for three days - Mahasaptami, Mahaastami, Mahanavami. The puja rituals are long and very detailed and complicated. Three days of Mantras and Shlokas and Arati and offerings - needs an expert priest to do this kind of Puja. Because of these facts, the number of Pujas held in the family has reduced and Durga Puja has mostly emerged as a community festival.
According to Hindu mythology a demon named Mahishasura, earned the favor of Lord Shiva after a long and hard penance. Lord Shiva, impressed with his devotion, blessed him that no man or deity would be able to kill him and that only a woman can kill him. Mahishasur was very pleased with this boon as he thought that a woman can never defeat him. Arrogant Mahishasura started his reign of terror over the Universe and people were killed mercilessly. He even attacked the abode of the gods and conquered the heavens and became their leader.
Today's most authentic form of the Durga is that of a ten handed goddess modeled out of clay astride a lion. Each of those hands carry a separate weapon in them except two, which holds the spear which has been struck into the chest of the demon, Mahishasura. The four children of the Goddess had also been added to the iconography - Laxmi, the goddess of wealth, Saraswati, the Goddess of knowledge, Kartik, the God of beauty as well as warfare and Ganesha, the 'Siddhidata' or the starter of everything in good sense.
Goddess Durga represents a united front of all Divine forces against the negative forces of evil and wickedness. The gods in heaven decided to create an all-powerful being to kill the demon king Mahishasur who was ready to attack them. At that very moment a stream of lightning dazzled forth from the mouths of Brahma, Vishnu and Mahesh and it turned into a beautiful, magnificent woman with ten hands. Then all the gods furnished her with their special weapons. The image of Durga, the Eternal Mother destroying the demon, Mahishasur is symbolic of the final confrontation of the spiritual urge of man with his baser passions.
During Durga Puja, God in the form of the Divine Mother is worshiped in Her various forms as Durga, Lakshmi and Saraswati. Though the Goddess is one, She is represented and worshiped in three different aspects. On the first three nights of the festival, Durga is worshiped. On the following three, Lakshmi and then Saraswati Devi on the last three nights. The following tenth day is called Vijayadasami. Vijaya means "victory", the victory over one's own minds that can come only when these three: Durga, Lakshmi, and Saraswati are worshiped.