The last five days of the month of Kartika are traditionally known as the Bhishma Panchaka. We observe it in honor of Grandfather Bhisma, who fasted in the last stage of his life, preparing to give up his life. In the Hari Bhakti Vilasa, it is said that if one is capable, one should observe fasting on the Bhishma-panchaka for the pleasure of the Lord. The fast should begin by remembering Bhismadeva on the day of the Utthana Ekadasi and should end on Purnima (the full moon]), which comes four days later (five days in total, including the Ekadasi), breaking the fasting in the late afternoon of the 5th day, after the appearance of the moon. The Padma Purana says that one pleases the Lord and makes spiritual advancement by such austerities.
The Padma Purana gives the following instructions on the observance of the Bhisma Panchaka, which were included by Srila Sanatana Goswami in the Hari Bhakti Vilasa:
arabhyaikadasim panca dinani vratamicaret
bhagavatpritaye bhismapancakam yadi saknuyat
The verse mentions that starting from the Ekadasi day (arabhya ekadashīm), one should observe a vow of austerity for five days (pañca dināni vratam-icaret) as an act of devotion to please the Lord (bhagavat-prītaye) in honor of the five days of Bhisma (bhishma panchakam). The verse adds that one should do it if able (yadi shaknuyāt), and Srila Sanatana Goswami adds the words kamyatvamiti dik (if one has the desire), emphasizing that this is an optional vow that one can observe or not according to his desire, although it is strongly recommended in the sense that one can obtain great merit by observing it since these days are so dear to the Lord. These five days are considered extremely auspicious to free us from our past karma and increase our devotion to Krsna.
Srila Prabhupada explains that such occasions are just like promotions when the gods in the store of Bhakti are available at a great discount. Devotees naturally buy at the store all year long, but still, such special dates are an opportunity for getting special blessings and advancing faster.
Any type of spiritual activities that we execute during the five days of the Bhisma Panchaka award great benefits. One can chant more rounds of japa, read more, listen to more lectures, and so on. Many spiritual masters recommend fasting on this occasion to obtain special blessings.
There are different levels of fasting one can observe on these five days. One can eat only ravish (kichiri without spices), can eat like on Ekadasi days, without grains, can eat only vegetables, fruits, and milk, and so on. Many devotees opt for fasting completely on these five days, taking only water (and prescription medicines), this is naturally much harder than just abstaining from grains, but it can bring great spiritual benefits.
When we speak about the Bhisma Panchaka, a controversy often emerges about the chronology of the dates. Many argue that observing the Bhisma Panchaka in the last days of Kartika is not correct, calculating it on the basis of the date of Gita-Jayanti and other days. There is one point to consider, however: spiritual life is not just about chronological events.
Time in the spiritual world doesn’t work in chronological order. There are infinite pastimes and they happen eternally. Even the events that happen in this material world connected with Krsna pastimes repeat infinitely in other universes. Bhisma Pancaka is not about a chronological date, but about remembering the passing of Bhismadeva in the context of the month of Kartik. For Vaishnavas, Kartik is all about surrendering to Krsna, and the passing of Bhismadeva is the highest example of that. Similarly, we also remember the pastime of Gadjendra. It’s about spiritual connection, not about chronological connection. Arguments about dates and chronological order are purely mundane.
In other words, the main point about Vaishnava festivals is the spiritual connection with great personalities or important spiritual events, and such connection is achieved by remembering such personalities and events, and not just a calendar date.
There are many verses in the scripture that describe the glories of the Bhisma Panchaka, starting with the Padma Purana and Skanda Purana, and culminating with the Hari Bhakti Vilasa and other books written specifically for Vaishnavas. Here are a few other references:
kartika vratinam vipra yathokta karinam naram
yama dutah palayante simham drstva yatha gajah
srestham visnu vipra tat tulya na satam makhah
krtva fratum urje svargyam vaikuntham kartika vrati
“If anyone fasts and observes the Kartika-vrata according to the rules and regulations, the Yamadutas, the messengers of Yamaraja, run away from him, just as an elephant runs away by seeing a lion. This dear fast [vrata] of Lord Visnu is even better than performing one hundred great sacrifices that would take him to heaven because the person who observes the Kartika-vrata goes to the spiritual world.” (Padma Purana)
sulabha mathura bhumau praty abdam kartikas tatha
tathapi samsarantiha nara mudha bhavambudhau
yani sarvani tirthani nada nadyah saransi ca
kartika nivasanty atra mathure sarva mandale
“Although one can easily reach Mathura on this earth, and although Kartika month is easily observable, and although in the month of Kartika all of the places of pilgrimage, oceans, rivers, and lakes come to the Mathura area, still, those human beings who are foolish and suffering in the ocean of material existence do not take advantage of it.” (Padma Purana)
Other verses from the Padma Purana describe:
“This liberating fast is unknown even to the demigods. If one listens to what I have said and fasts on these five days, even if he had sex with his sister, daughter, mother, etc. he becomes free from that sin. Even if someone attentively listens to this narration, he also achieves happiness. I tell you the truth again and again that one should keep this fast secretively, and continue performing it unceasingly – such a person surely achieves liberation.” (Uttara Khanda)
“In the month of Kartika, which is very dear to Sri Hari, one who bathes early in the morning attains the merit of bathing in all places of pilgrimage. Anybody who offers to the Lord a ghee lamp in the month of Kartika, O brahmana, becomes free from all kinds of sins, such as killing a brahmana, and he goes to the abode of Lord Hari.” (Brahma Khanda).
There a two other verses from the Hari Bhakti Vilasa:
“This five-day fast is difficult for those people who are not serious in Bhakti. If someone observes this fast, he is considered to have observed all other fasts, because it is greatly meritorious and frees one from great sins. Therefore, everyone should make a great endeavor to fast on these last five days of Bhismadeva.”
“The fast should begin by remembering Bhismadeva on the Ekadasi day and should end on Purnima [the full moon]. One should eat only vegetables or fruits and should worship Lord Gauranga Krishna. If he needs to eat, he should do so only in the evening. A woman can also observe this fast by the suggestion of a twice-born, and if a widow observes this fast, she can also attain liberation and happiness.”