According to Ayur Veda (a system of Hindu traditional medicine of Vedic tradition) every person, regardless of age and gender, viz., old, young, pregnant and sickly persons, should take regular oil bath, i.e., every week. "Oil Bath" is known in Sanskrit as "Thaila Snanam". The Hindu scriptures recommend that one should take oil bath on lunar days, viz., Saturday, Monday, Wednesday and Friday. And, should strictly solar days, viz., Sunday, Tuesday and Thursday.
The scriptures say that Saturdays and Fridays are particularly beneficial for males and females respectively. In addition, the following days are forbidden too: New Moon, Full Moon and one's Nakshatra (=star).
When males take oil bath, i.e., as they are applying oil on their heads and bodies, they are to recite the following mantra, invoking the blessings of seven "chiranjeevis" (= known for their longevity and perfect health): Ashwathama, Bali, Vyasa, Hanuman, Vibheeshana, Krupa and Parashurama:
Ashwthamo Balir Vyaso Hanumanshcha VibhishaNah,
Krupah Parashuramashcha,Saptaite Chiranjivinaah.
Females are to chant the following mantras:
Ahalya Draupadi Sita Tara Mandodari tatha
Panchakanyaah smarennityam mahaapataka naashanam.
The five characters in the mantra are Ahalya, Draupadi, Sita, Tara and Mandodari, the "Pancha kanyas", whose blessings will destroy all sins.
By nature, thaila snanam (=oil bath) will make one sleepy. For this reason, therefore, astrological texts and the scriptures have prescribed only some days. That is to say, taking the bath on these days will mitigate this soporific effect. But, first of all, if oil bath should be healthful and beneficial, why does it make one somnolent and drowsy?
Ayur Veda says that the vital energies in our body, vatha, kapah, pitha, must be balanced. When one takes oil bath, the balance of these is upset in the body: one feels sleepy. Nonetheless, as it has been mentioned earlier, by taking the bath only on specified days, this natural imbalance is addressed.
Other than the days mentioned in paragraph one, "Chathurthasi" is another day that is ideal for oil bath for both genders. So is Friday afternoon. Saturn, who is in Sanskrit as "ayush kaarakan", rules Chathurthasi (=rikhtha thithi) and Saturday. Indira rules Friday, which the scriptures have declared, benefits everyone.
Note: Oil bath on Saturdays can help appease Saniswaran, and reduce His bad effects during His 7 1/2 testing period.
According to Bharadvaja, who is one of the seven great rishies in this manvantra, oil bath should be taken on the tithis and the days that have prescribed.
Oil Bath on Deepavali
In the dark fortnight of Ashwina month, on the fourteenth day, early in the morning, one should take oil bath, i.e., applied oil on the body and cleanse it with lukewarm water. If ashwaja krishna chadurthasi [Narakachaturthi day] falls on a Sunday, however, when the star Swathi is ruling, scented oil bath should be taken.
On Deepavali day, Goddess Lakshmi lives in oil, and Mother Ganga lives in warm water. This bath removes bad luck. After the bath, one ought to wear new clothes and jewellery.
Note: Generally, the scriptures forbid taking oil bath on holy days. However, Deepavali is an exception. This exception has been provided for by Sri Krishna who granted Nakarasura's boon that everyone, in remembrance of his death, should take an oil bath on Deepavali.