Oil bath - benefits

Why?
Gingely oil on applied on skin fecilitates the production of pro vitamin D. 
Following is the transformation path of the Provitamin D:
Provitamin D( Type: cholesterol) produced by skin when exposed to UV rays
Previtamin D3 on further exposure 
Vitamin D in warm
Calcediol in liver
Calcetrio hormone in kidney(major part in calcium metabolism and an anti-cancer agent).

If enough vitamin d is produced the even a glass of milk a day is enought for strengthening bones( can avoid the supplements for it ) .

When not to?
Oil bath basically makes you body cool and hence logically is better to avoid oil bath on the following days as our body already becomes cool :
Sunday,
Monday
Tamil new year
New tamil month,
Full moon,
New moon
Eclipse 
Also don't sleep during day of oil bath  .

References: http://www.uvguide.co.uk/vitdpathway.htm