Navarathiri Pooja is not a pooja that is done only for one day. It is done for 9 days. Nava means "nine". Since, the pooja is done for 9 days, the pooja is so called as "Navarathiri" Pooja. This pooja is mainly done towards the Goddess - Shakthi and she is the main deity for this pooja.
This 9 days of pooja is segregated into 3 parts and each parts contains 3 days of each. Therefore, in these 9 days, first 3 days of pooja is for Sri Durga Devi, the next 3 days, pooja needs to be done for Sri Lakshmi and the final 3 days for Sri Saraswathi. Further one more day the pooja is extended and on that particular day, we have to do pooja for all these 3 deities.
About the main deity:
Goddess Saraswathi is referred as "Gnana Shakthi", Goddess Sri Lakshmi is referred to as "Kriya Shakthi" and Goddess Parvathi is called as "Iccha Shakthi". All these 3 Shakthis joined together as "Chandika Devi" to attain a common goal ie killing Mageeshan.
Story behind the Pooja:
Sri Devi Parasakthi has taken lots of Avathaars (incarnation) to destroy the evil. And, the one among them is the avathaar of Mahishasura Mardini. Now, let us know the reason why the avathaar was taken.
Once, there lived two Arakkans (demon) by named Rambhan and Surambhan. They did a severe penance, Devendran Indran, changed himself to a crocodile and pulled the legs of Surambhan and killed him. On seeing this, Rambhan got angry and finally thought of killing himself and dedicating his life to do the Ukrathavam, Agni Bhagavan appeared infront of him and asked for his wish.
Rambhan asked that he should be blessed with a boon that will make him to destroy all the Chatrus, Devars and conquer them. He also added that he should be blessed in such a way that he can take any kind of form which he thinks of. And at the same time, he should be blessed with a child with all these Characters.
Agni devan gave the varam for which he opted for and added that he can get a child with anyone he can mate with.
Rambhan went towards the forest where Yakshars lived and on the way, he saw a lady buffaloe and took it towards the Bhadala Lokam and soon the buffaloe started tried to kill the lady buffaloe and at that time, Rambhan came there to help but accidentally, he was killed during that time. Finally, all the Yakshars started to fire the body of Rambhan.
At that time the lady buffalo jumped towards the fire and killed itself. During that time, an Asuran with the head of buffalo came out from the fire and was named as "Mageeshan".
Mageeshan did a strong penance in Mearu mountain for lots of years towards Brahma devan fully satisfied on the tapas of Mageeshan, Brahma devan appeared before him and asked which varam (boon) he wants for. He asked that he should live in this earth forever but brahma devan refused to give this varam because all the Jeevathmas that are born in this world should die one day. As an alternative to this varam, he asked that he should not be killed by an ordinary human or by Devars but only should be killed by a lady. Brahma devan gave this varam and disappeared from their.
Having got the Varam, Mageeshan went towards Swargham to conquer it. Both Mageeshan and his war troop fought against Indiran and his war troop for lots of years and finally, Indiran was defeated and conquered the Swargha Lokam. Conquering the Swarga lokam, he went towards the Surya Lokam, then to Vaayu Mandalam, then went to Chandra Lokam, then to Yama Lokam and conquered all of these Lokams thereby riding all of them from their places.
All of the devars and rishis seeked the help of Brahma devan and he explained that the time for the life of Mageeshan to end has come but at the same time he made all of them to remember that he would be killed only by a lady.
Knowing this, Lord Shivan and Sriman Narayanan got angry and their anger fire along with fire in the minds of Devars and Saadhu's mixed together and from that, Chandika Devi emerged.
Lord Shivan gave his shoolayudham, Sriman Narayanan gave his Chakrayudham, Devendran Indiran gave his Vajrayudham, Yaman gave his Dhandayudham, Vaayu gave his Bow and Arrow, Lord Brahma gave his pure clothes and Samuthira Raja gave Garlands and Imavaan gave his Lion as the Vaghanam (Vehicle) to Chandika Devi who prepared herself to make a war against Mageeshan.
There was a great war between Chandika Devi and Mageeshan. Devi killed all the Asuras and marched towards Mageeshan to fight against him. As the varam got by his father, Mageeshan changed himself to a lion and when Devi took the knife to cut his head, he changed himself to an elephant. As his trunk was cut, he changed himself back to the Roopam of Mageeshan with the head of Buffaloe. Finally Chandika devi jumped towards him and smashed his buffaloe head and killed him.
All the Devars spread flowers on her and the trimurthies gave their blessings to her.
The time in which Chandika Devi incarnated as Mageeshasura Mardhini is said to be the festival that is celebrated in Purattasi month (October - November) starting from the Amacaasai for 10 days. During these 10 days Durga devi, Lakshmi Devi and Saraswathi Devi are kept as the main deities and should be done poojas for 3 days each. And on the 10th day, all of these 3 deities are kept together and should do the pooja to complete the Navarathiri pooja.
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