सर्व मङ्गल माङ्गल्ये शिवे सर्वार्थसाधिके। शरण्ये त्र्यम्बके गौरि नारायणि नमोऽस्तु ते॥
Fast does not mean ‘not eating’. Rather it basically means controlling the desires and simultaneously cultivating positive mental attitudes. Desires can be of many types: to eat tasty food, smell, listen to a particular music, lust etc.
Fast, therefore, can be of many types. Food fast means controlling the desires for food items which you otherwise may not resist eating.
In Navratri, during the first three days, a person is required to indulge in activities which reduce the negativity in the body.
The physical purification involves regular bath (of the external body as well as various orifices); mental bath which involves self-confession exercises, and willful attempts not to think negative.
During these days, a person tries to restrain himself/herself from thinking, speaking or doing any activity which can harm another person. These negative thoughts should also not be suppressed or repressed as otherwise, they will manifest into some internal diseases.
The next three days, after the negativity in the mind, has been reduced, involves building positivity in the mind by way of various positive mental exercises. The last three days involve reading and understanding good scriptures and learning from the sins of others.
Once you have bid goodbye to negativity in the first three days (representing Kali) and have built up happiness and positivity in the mind (representing Lakshmi) the only thing left is to acquire spiritual knowledge (representing Saraswati).
During these last three days, a person is purified enough to understand and grasp the knowledge of ‘Self’ and to understands and discriminate between good and bad.
After the nine days of self-discipline is complete, the person acquires inner happiness which is nothing but one’s exposure or appointment with the true self or the consciousness (Rama). That is what Dussehra is with the killing of Tamas (Kumbhakarana), Rajas (Meghnath) and ego (Ravana).
The same interpretation lies in the Navratri ending up with Dussehra – the win over the ego and attaining inner happiness.