Diwali is called the 'Festival of Light' in India, but it is more appropriate to think of it as more like a New Year of Luck and Wealth Celebration. Diwali comes on the New Moon of the month of Kartika (usually in late October).
But that is just a surface explanation. Central to Indian philosophy is the idea that there is a deeper level of existence beyond the physical body and mind. That level is pure, infinite, wise and loving. It is called the ParamAtman. This ParamAtman is your true nature. It is the same true nature that is at the heart over every being. Thus, it is not YOUR true nature is THE true nature. All beings are joined in one-ness to the ParamAtman - The Supreme Self.
The only reason we suffer in life is because we mistake ourselves to be the denser forms of body, feeling and thought - forgetting our trues selves as eternal, loving spirit. This idea is at the heart of every spiritual tradition.
So, each year - at Diwali - we celebrate the birth of our physical being as an expression of the divine will. Diwali is the celebration of this Inner Light. It is the time to realize the inner effulgence that has the power to outshine all darkness within. It is the time to realize all of our external longings and accomplishments to merely be a path to the divine within, and re align our commitments with the highest truth of Self.

Happy Diwali ONE and All!