The last 5 days of the month of Karthika (October-November) are traditionally known as the Bhishma Panchaka or the Vishnu Panchaka. This is to commemorate the last five days in the life of Bhishma.
Bhishma is also known as one of twelve mahajanas—authorities—in the science of devotional service to Krishna. In his youth, he received the benediction that he would only die when he wished to, and he spent his final few days giving spiritual and political instructions to King Yudhishthira while lying on a bed of arrows, mortally wounded, on the battlefield of Kurukshetra.
In the Hari Bhakti Vilasa, it is said that if one is capable, one should observe fasting from certain foodstuffs on the Bhishma Panchaka for the pleasure of the Lord. This is optional. The Padma Purana says that one pleases the Lord and makes spiritual advancement by such austerities.