Diwali is one festival that can be described as truly India. Its a festival that is celebrated by most Indian religions. The reason why we can it the festival of lights is because Indians light clay lamps called "diyas" all around the house to direct the Goddess Lakshmi to the home and give be blessings. You can read more about Diwali the festival of lights here. If you wanted to visit India just for Diwali, you might wish to take a note of the date of Diwali as it varies according to the cycle of the moon.