Why do we Lit Lamps / Deepalu During the Month

'Deeparadhana' or lighting the lamps during not only this month but it is observed daily in any puja or Hindu ritual. According to many Puranas lighting Diyas shows purity, but there is also  a natural side of benefits for our health and for environment (acts like source of energy light and as a germicide). Diyas symbolizes to drive away of ignorance and awakening of the divine light within us.
According to Puranas:

 According to many Puranas   diyas  allow us to get rid of  all the impurities but provides us with the greatest form of energy, light.

Lamps or diya are  also lit during offering Aarti at the end of any puja. It symbolizes the divine aura and also helps devotees to have a clear glimpse at the idol of the deity.We generally take Aarthi  by moving our hands around the flame  and hence believe to receive the divine energy of the God into us. It reflects the siddhanta of Aatma and Paramatma. 

Aarthi

Why we lit   diyas during Karthika Mas :

"One who Lit a lamp in Karthika Masam will have the blessings of  their ancestors (Whom we call as Pitrudevatulu)  along with Lord Shiva and Lord Vishnu."

Diyas  can  be made of Mud   or   with rice flour  or a silver one, important thing is that we should be   using a new one daily.It is also said that using Steel diyas may end up in poverty .

Rice Flour Diyas

Religious Particularity of Diyas:

Religious belief even explain why all Hindus lit diyas while offerings and conducting pujas as it symbolizes the energy of Goddess Lakshmi which keeps us away from darkness of Foolishness  and welcomes us into the world of ‘ Knowledge’ (light). Another interesting theory is that the three colors of diya (white,Red and blue) represent Goddess  laxmi,Sarswathi and Parvathi