In essence Durga is the epitome of strength or Shakti which is the female companion of Shiva. In other words Shiva's strength comes from Shakti. Durga is also another form of Parvati the consort of Shiva just as Shiva also has many forms such as Tandava and Rudra. See the various forms of Shiva and Parvati in the 51 Pithas of Shakti, in the Hindu Pilgrimage page. This page can be accessed from the main Hindu Calendar page.
Shiva and Parvati live in the adobe of Himalayas on Mount Kailash which is near Nepal. They have four children Saraswati, Lakchmi, Ganesha and Kartika. Saraswati is the Goddess of learning, music and the fine arts. Lakchmi is the Goddess of wealth. Ganesha is the God of intellect and knowledge. Kartika is the God of warfare. Occasionally Shiva or Parvati come down to Earth to help mankind or slay a demon.
Such was the occasion when Durga made the journey from Kailash to slay the demon Mahishasur who was harassing people on Earth. Mahisha means a water Buffalo and Asura means a demon but it literally means the one who is without Sura or did not drink the nectar of immortal life during the Samudra Manthan.
This demon Mahishasur could hide among herds of Buffalo and harass people and hinder yagnas.
Durga came to Earth on the invitation and prayers of the people who were being harassed . She came down in her form of Mahisha Mardini or the slayer of the Buffalo which has ten hands holding ten different weapons given to her by the various Gods. She came riding her mount the Lion on the invitiation of her devotees. On the eighth day of the Sukla Pakcha in the month of Aswin she arrived on Earth. On the ninth day the battle ensued with Mahishasur and the battle continued through the night. The demon hid easily among Buffaloes by taking their form . But Durga cut off its head to reveal the Demon and eventually on the tenth day or on Vijay dasami she slayed Mahishasur with the help of her mount the Lion. This event is celebrated every year as.
The worship basically begins with the singing of Mahalaya or the invitation from her devotees on the Amabashya or the new Moon of Ashwina which usually comes around September-October. After which the worship of nine nights and ten days begin. The actual installation and worship of theidols of Durga along with the idols of her children Sarasvati, Lakchmi, Ganesha and Kartik are worshipped begins on Panchami of Sukla Pakcha. First Sarasvati is worshipped on Panchami, then Lakchmi is worshipped on Shasthi who are also considered different forms of Parvati and then Durga is worshipped on Saptami. On the eighth day the puja of Mahasthami is held when she actually arrives from Kailash and enters the clay image. Much fanfare is held to announce her arrival on this day including the playing of drums, cymbals and conch sehlss and a great worhsip is conducted. During the puja cermonies Mantras are recited by priests and devotees asking the Goddess to give knowledge, wealth and courage to all. On the ninth day the great battle with Mahishasur ensues and is celebrated as Mahanavami. On the tenth day victory over the demon is celebrated by the puja of Vijay Dasami. After the ceremonies the clay images are floated into nearby rivers and the ceremony is called Visarjan. People celebrate the vcitory of good over evil by eating sweets and greeting their neighbours.
The festival is held in the autumn equinox when the days and nights become equal after which the days become shorter than the nights. This is also a period of harvest in the Northern hemisphere.
However some Hindus also believe that the slaying of the demon using ten instruments and ten hands signifies our internal struggles over evil felings that we accumulate over the year such as jealousy and anger and envy. These bad feelings are vanquished during the worship and we emerge pure and happy again. The ten instruments in the hands of the Devi are needed to hunt the ill feelings produced by our ten sensual organs: two eyes, two nostrils, two ears, the mouth, the touch of the skin, the sexual organ and the anus.