Diwali, being the festival of lights, thousands of lamps are lit in and outside every home on the day.
Lamp or Deep is the symbol of knowledge. Lighting the lamp of knowledge within us means to understand and reflect upon the significant purpose of each of the five days of festivities and to bring those thoughts in to our day to day lives. Kalpasutra mentions that the King Chetaka of Vaishali with several confederate kings, had a great lightning of lights, since they said:
"since the light of intelligence (Vardhamana Mahavira) is gone, let us make an illumination of the material matter"