The tradition is that one of the goddesses was in a battle which lasted for 9 days. The enemy would only come out at night, so for the 9 days, people would dance during the night to give the goddess energy.
10:30 P.M.

12:30 A.M.

3:00 A.M.

After the 9 nights, the enemy, Ravana was defeated.
Click here to see Ravana being blown up on the 10th night.