Dashain is the 15-day national (religious) festival of Nepal, It is the longest and the most auspicious festival in the Nepalese annual calendar, celebrated with great rejoice by Nepalese of all caste and creed throughout the globe. It is not only the longest festival of the country but is also the one which is most anticipated. The festival falls around September–October, starting from the bright lunar fortnight and ending on the day of full moon. Dasain is also popularly referred to as Bada Dasain, Dashera, Vijaya Dashami etc.
Throughout the country the goddess Durga in all her manifestations are worshiped with innumerable pujas, abundant offerings and thousands of animal sacrifices for the ritual of holy bathing, thus drenching the goddess for days in blood. This festival is also known for its emphasis on the family gatherings, as well as on a renewal of community ties. People return from all parts of the world, as well as different parts of the country, to celebrate together. All government offices, educational institutions and other offices remain closed during the festival period.
Dashain commemorates the victories of the god and goddesses over the demons. One reason it is celebrated is because Lord Ram killed Ravana, the king of demons during this time. It is believed that lord Ram was able to win the battle only because he was blessed by goddess Durga. However the main celebration symbolizes the victory of the good over the evil. Mahishasura, a demon, had created terror in the dev-lok (the world of gods). All the gods and saints prayed to the Adi-Shakti in order to kill Mahishasura, but when nothing happened, Goddess Durga finally emerged and massacred the demon thus saving everyone from terror.
The first nine days of Dashain symbolizes the battle which took place between the different manifestations of goddess Durga and the demon Mahishasura. The tenth day is the day when Durga finally defeated Mahishasura. Goddess Durga is worshipped throughout the country as divine mother goddess.
Throughout the festival people pay homage to the various forms of the Supreme Goddess, Durga. The festival is important since it reminds everyone of the universal principles of truth, justice and virtue that must prevail over deception, injustice and wickedness. It is believed that if she is worshiped properly and pleased then good luck is bound to happen. However, if the goddess is angered through negligence then misfortunes are said to happen.
Dashain is the biggest festival for the Hindus. The followers of Shakta cult take it as the day of falling of demon Mahishasur by goddess Durga. For non-Shakta Hindus, this festival symbolizes the victory of Rama over Ravana, the characters of the epic Ramayana. The Buddhists remember this day as Emperor Ashoka of the Indian subcontinent abandoned violence on this day and entered the path of Buddhism.
In the Kathmandu Valley, among the Newars, the festival is known as "Mohanee", with slight difference in rituals and significance, thus more than often confused with the Dasain.
Information based on http://en.wikipedia.org/wiki/Dashain