The last 5 days of the month of Kaartik are traditionally known as the Bheeshm Panchak or the Vishnu Panchak. If one is capable of, one should observe fasting from certain foodstuffs on these days. The Padm Puraan says that one pleases the Lord and makes spiritual advancement by such austerities. The last 5 days of the month of Kaartik are traditionally known as the Bheeshm Panchak or the Vishnu Panchak. It is said that if one is capable, one should observe fasting from certain foodstuffs on the Bheeshm Panchak days for the pleasure of the Lord. The Padm Puraan says that one pleases the Lord and makes spiritual advancement by such austerities.

One may keep the fast drinking only fruit juices, or milk, or yogurt or eating Havishya, or eating once a day.

Fast-breaking should be done on the last day of the Bheeshm Panchak or the next day? and how to break fast? Padm Puraan says that on the fifth day of the Bheeshm Panchak in the night one can break the fast by taking fruits, roots or 'Havishya' (unspiced Khichadee).
[Padm Puraan]    [ Agni Puraan, 23]
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In Padm Puraan it only stated that in addition to Ekaadashee one can observe a "Pancha Raatrikaa" fast until Amaavasyaa also. Here are offerings prescribed for Bheeshm Panchak.