What is "panchaka"

The last five days of the month of Kaartika are traditionally known as the Bhishma Panchaka or the Vaka Panchaka. According to Oriya Ramayana written by Balarama Das it is said that if one is capable, one should observe fasting from certain foodstuffs on the Bhishma-panchaka for the pleasure of the Vishnu. The Padma Purana say that one pleases Vishnu and makes spiritual advancement.