why navratri is celebrated ? what is the story behind navratri ?

It's navratri 

Nav means "nine" and ratri means "night " 

This  festival ends with what's known as Dussehra, which is the celebration of good over evil.

"The story bhind navratri "

Navratri is a time when Hindus celebrate the goddess Durga for killing the demon, Mahishasura.

A more powerful God, Lord Brahma, had blessed Mahishasura with the gift of immortality because of his dedication to him, which meant he could never die.

However, this gift did come with one condition - the only person who would be able to defeat Mahishasura would be a woman.

Mahishasura didn't think any women would be strong enough to kill him and was happy with the deal.

Over the years, Mahishasura and his men became a powerful force attacking people on Earth and no gods were able to defeat him.

Finally Lord Brahma, Lord Vishnu and Lord Shiva - the three most powerful Hindu gods - decided to come together and create the goddess Durga, a powerful woman whose job would be to destroy Mahishasura.

The gods equipped the new goddess Durga with lots of weapons before she went into battle with Mahishasura.

Durga fought the demon for 10 days, and it proved quite difficult as he kept changing his form to confuse the goddess. However, when he finally turned into a buffalo, Durga was able to defeat him.