Fasting

Bhishma said thus:


“O Dharmaja! in the past, I also asked Saint Angirasa( First Guru of  Shukra and  Brihaspati ) in the same manner. I told you what he told me.

The following are the days auspicious for observing Upavasa. They are.....

1. full moon day,
2. Sukla Ashtami,
3. Sukla Panchami,
4. Shashti,
5. Bahula Pamchami,
6. Bahula shashti.

Saint Vishvamitra observed one time meal during his entire life. By observing Upavasa, body and mind become pure.

Detail
http://www.sacred-texts.com/hin/m13/m13b068.htm