Firstly, this earth is not a lonely inhabitated planet in the cosmic space. In fact, this earth is the newest of all the inhabited planets in the universe. After this earth got separated from the Sun, the earth slowly cooled down to the climatic conditions that could sustain growth of organisms.
During this period, the pioneers came to this earth from other planets in the Universe. There were precisely 18 such pioneers who came to this earth and hence they were known as Pathinen Siddhars (18 Siddhars or 18 Masters.) These 18 Siddhars came to this earth alongwith their inter-cosmic assitants (Pathinettaampadi Karuppu in Tamil) from other inhabited planets of this universe.
Secondly, these 18 Siddhars researched the conditions of this newer earth and worked in numerous ways to evolve the living creatures from one cell to huge animals. After the highest living beings got evolved, these 18 Siddhars and their inter-cosmic assitants worked with these animal-like beings. Through this hardwork they evolved human beings out of the animals using language as a tool to speak and think. The first language that they used was Tamil. That is why Tamil is the most ancient language and also the mother of all other languages of this earth.
Thirdly, these 18 Siddhars took almost 4,85,000 earth years to evolve thinking human beings. So, they had to return back to their planets and come again many times to this earth in the course of this evolution process. Their hard work yielded results finally to help them form a kingdom. The Social Philosophy they used for this is called as Indhu Philosophy. This Indhu Philosophy has been mixed up with the habits of incoming foreigners to this India 3000 years back and became Hindu Philosophy. The Westerners learnt about this Hindu Philosophy only and they are yet to know about the true Indhu Philosophy.
The 18 Siddhars developed the Individual life of human beings. After that they made the individual human beings lead a Family Life and then Social Life. The blood related individual human beings get tied up together in a family as father, mother, sons and daughters. Many families together make a society. Grouping such societies together, the kingdom or government got formed.
The day these 18 Siddhars came to this earth has been considered as a significant day for this planet and that day is being celebrated as Deepavali even today. In this long duration of 43,73,116 years (43,73,117th year as of 2016 CE) many many incidents have happened coinciding with this Deepavali day. So, many groups of people celebrate this day owing to different incidents they consider significant. Killing of Narakasura was one among these numerous incidents that took place on this Deepavali day.
The main purpose behind instituting a celebration on this Deepavali day was to remember fore-fathers and give offerings to them. Since the 18 Siddhars came from older planets in the Cosmic space, the link between this earth and other solar systems are remembered by lighting lamps all over in the early morning before dawn. That is why this day was named as Thee (Light) + Aavali (in line formation) day. This name has become Theevaali or Deepavali.
So, in essence, this Theevaali celebration is actually Tamils’ Festival day. People speaking other languages in India started celebrating after seeing the Tamils celebrate this day.
Taken from the writings of His Holiness Siddhar Arasayogi Karuwooraar, the 12th Pathinensiddhar Peedaathipathi.