Dashain commemorates a great victory of the gods over the wicked demons. One of the victory stories told is the Ramayan, where the lord Ram after a big struggle slaughtered Ravana, the fiendish king of demons. It is said that lord Ram was successful in the battle only when goddess Durga was evoked. The main celebration glorifies the triumph of good over evil and is symbolized by goddess Durga slaying the terrible demon Mahisasur, who terrorised the earth in the guise of a brutal water buffalo. The first nine days signify the nine days of ferrous battle between goddess Durga and the demon Mahisasur. The tenth day is the day when Mahisasur was slain and the last five days symbolise the celebration of the victory with the blessing of the goddess. Dashain is celebrated with great rejoice, and goddess Durga is worshiped throughout the kingdom as the divine mother goddess.
Source: Visit Nepal