Goddess Durga represents a united front of all Divine forces against the negative forces of evil and wickedness. The gods in heaven decided to create an all-powerful being to kill the demon king Mahishasur who was ready to attack them.
At that very moment a stream of lightning dazzled forth from the mouths of Brahma, Vishnu and Mahesh and it turned into a beautiful, magnificent woman with ten hands. Then all the gods furnished her with their special weapons. The image of Durga, the Eternal Mother destroying the demon, Mahishasur is symbolic of the final confrontation of the spiritual urge of man with his baser passions.
As per our great epic Mahabharat, Pandavas after wandering in the forest for 12 years, hung their weapons on a Shami tree before entering the court of king Virat to spend the last one year in disguise. After the completion of that year on Vijayadashmi the day of Dassera they brought down the weapons from the Shami tree and declared their true identity. Since that day the exchange of Shami leaves on Dassera day became symbols of good, will and victory.
This festival has immense mythological significance. As per Ramayan, Ram performed "chandi-puja" and invoked the blessings of Durga to kill Ravana, the ten-headed king of Lanka who had abducted Seeta and had charmed life. In order to worship Goddess Durga, Lord Rama needed 108 blue lotus flowers. However, he could manage only with 107. To attain the magical number, he decided to offer one of his eyes, which was lotus-shaped and blue in color, at the Goddess's feet. Satisfied with his devotion Goddess Durga appeared and blessed him. Durga divulged the secret to Ram how he could kill Ravana. Then after vanquishing him, Ram with Seeta and Laxman returned victorious to his kingdom of Ayodhya on Diwali day.
Kautsa, the young son of Devdatt, insisted on his guru Varatantu to accept "gurudakshina", after finishing his education. After lots of persistence his Guru, finally asked for 14 crore gold coins, one crore for each of the 14 sciences he taught Kautsa. Kautsa went to king Raghuraj, who was known for his genorisity and was an ancestor of Rama. But just at that time he had emptied all his coffers on the Brahmins, after performing the Vishvajit sacrifice. So, the king went to Lord Indra and asked for the gold coins. Indra summoned Kuber, the god of wealth. Indra told Kuber, "Make a rain of gold coins fall on the "shanu" and "apati" trees round Raghuraja's city of Ayodhya." The rain of coins began to fall. The king Raghu gave all the coins to Kautsa, who gave 14 crores gold coins to his guru. The remaining coins were lavishly distributed to the people of Ayodhya city. This happened on the day of Dussehra. In remembrance of this event the custom is kept of looting the leaves of the "apati" trees and people present each other these leaves as "sone" (gold).