Long ago there was a demon called Mahishasura. He was given this name as he was in the form of a bison. Being a powerful demon, Mahishasura wanted to conquer all the 3 worlds in the universe. He began to pray for Brahma to acquire a boom to fulfill his desire as his prayers continued for years, Brahma had to appear in front of Mahishasura. 'No man in this universe should be able to kill me, was the boon obtained by Mahishasura from Brahma deva. 

Goddess Durga defeated Mahishasura

After getting boon Mahishasura started torturing everyone. Even Brahma, Vishnu, and Shiva were not able to restrain him. So they thought of finding ways to defeat Mahishasura. As there was a chance for women to defeat Mahishasura they wished to create a woman so powerful as to defeat him. They combined their power and created Goddess Durga, they gifted their weaponry to her who was ready to fight with Mahishasura with only 2 but 18 hands. The weapons she got from God were. With Sudarshan chakra from Lord Vishnu, Trident(Trishul) from Lord Shiva, and Vajrayudha from Lord Indra she mounted on Lion and started a battle with Mahishasura. Mahishasura likewise was ready, he attacked Goddess Durga with millions of soldiers. 

But Durga and her lion turned down the army of Mahishasura. After a fierce battle that lasted for nine days, Goddess Durga finally killed the demon Mahishasura. The gods celebrated this occasion which is now continued as Vijayadashami. 

The battle between Goddess Durga and demon Mahishasura Continued for 9 nights. the festival days of Vijayadashmi are called Navaratri. As the victory was achieved on the 10th day it was called Dusshera. Dusshera is celebrated as the 10th day of the Hindu month of Ashvin.

Here is the list of 7 facts that everyone should know about Navaratri & Dusshera.

1. The word Navratri is a combination of two words Nav means nine and Ratri means Night.
2. Nine days of Navratri signify one of the nine forms of Goddess Shakti; Durga, Bhadrakali, Jagadamba, Annapurna, Sarvamangala, Bhairavi, Chandika, Latika, Bhavani and Mookambika
3. Garba and Dandiya are two traditional dance forms that originate from the state of Gujarat and it is trendy in India during the Navratri festival.
4. Mahishasura had the head of a buffalo so Navratri people sacrifice buffalo in many parts of the country. It should be beheaded in a single stroke.
5. In some parts of India, women plant nine different kinds of food grain seeds in small pots. The newly grown saplings are then offered to the Goddess.
6. On the ninth day of Navratri in Karnataka, people worship weapons.
7. In the southern part of India, in the state of Tamil Nadu, Dusshera is called Golu.

It is also said that on this day Lord Rama defeated King Ravan. Dusshera is celebrated to mark Lord Rama's victory over the Ravana.
On the 10th day i.e., Dusshera huge effigies of Ravana are burnt with fireworks which symbolize the quelling of darkness by light. 
Dusshera also brings joy on the way to the next big festival called Diwali. Diwali is celebrated 20 days after Dusshera and signifies the return of Lord Rama, Sita, and Laxman to Ayodhya after defeating Ravana.

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