Durga Puja symbolizes the ultimate victory of good over evil. The Mother Goddess also represents unification as she was created by drawing positive energy from Trimurthy (Bramha, Vishnu, Shiva) and many other Gods and Goddesses, to destroy Mahishasura (Buffalo demon, representing evil). The event signifies the triumph of hope over despair and the rejuvenation of mind and spirit.
In Bengal, the event is primarily celebrated from Mahasashthi to Bijoya Dashami in the month of Ashwin (late September - October). The ritual spans up to ten days in different parts of India. There are innumerable articles in Internet exploring the history, religious and social significance of worshiping the Mother Goddess.
Few randomly chosen links are listed for additional information.