When it falls
Dussehra — also called Vijayadashami — falls on the tenth day (Dashami) of the bright fortnight of the lunar month of Ashwin, immediately after the nine nights of Navratri, usually in September or October.
Which deity is worshipped
Dussehra closes Navratri, the nine nights dedicated to Durga, and in eastern India it is above all her festival — the day her clay idols are ceremonially immersed in a river or sea after days of worship. In northern India, the same day is instead framed around Rama, whose victory over Ravana is re-enacted and celebrated as the day's central story.
Why it's celebrated
Two distinct victories are marked on the same date. The Devi Mahatmya tells of the buffalo demon Mahishasura, who had a boon making him unkillable by any man or god — a loophole the gods closed by creating Durga, who battled him for nine nights and beheaded him on the tenth, giving Navratri and Dussehra their shared name Vijayadashami, "the victory of the tenth day." The Ramayana tells the parallel story of Rama's war against the ten-headed demon king Ravana, who had abducted his wife Sita — a war that ends, after Hanuman and an army of vanaras help build a bridge to Lanka, with Ravana's defeat on this same tenth day. Both stories carry the same underlying idea: dharma prevailing over adharma, regardless of which specific battle is being retold.
How it's celebrated
In North India, the story of Rama is performed over the preceding days as Ramlila — a folk dramatization of the Ramayana staged in towns and neighbourhoods — culminating on Dussehra with the burning of huge effigies of Ravana, his brother Kumbhakarna and son Meghnad, packed with firecrackers. In West Bengal and much of eastern India, the day instead concludes Durga Puja: after days of elaborate pandal displays and worship, the idol is taken in procession (Bisarjan) and immersed in water, often with drumming, dancing and considerable public emotion at the goddess's departure. Across India, the day is also considered auspicious for starting new ventures, buying property or tools, and initiating children into learning (Vidyarambha).