In the month of Magha as per the Hindu calendar, on the day of Ekadasi, Bhishma Ekadasi is observed. It occurs on the eleventh day of the lunar fortnight. The month of Magha happens to fall in the period of January and February as per the English calendar.
The Bhishma Ekadasi falls in the Uttarayana Punyakalam which has many auspicious days dedicated to Bhishma like the Bhishma Ashtami and Bhishma Dwadasi. It is widely acknowledged that Bhishma chose this period of Uttarayana Punyakalam to leave his mortal coil and unite with the Supreme Soul. In the northern states of India on this particular day Jaya Ekadasi is observed.
The virtues of observing Bhishma Ekadasi and its special importance of linked to Lord Vishnu is mentioned in a number of puranas including the Haribhakti Vilas. The Padma Purana and Skanda Purana also detail the significance of Ekadasi in detail.
As per the puranas, it is believed that Bhishma revealed the Vishnu Sahasranama Stotram to the Pandavas in the presence of Lord Krishna, an incarnation of Lord Vishnu, on this day particular day while lying on the bed of arrows during the great war of Mahabharata. The Vishnu Sahastranama Stotram is a verse dedicated to Lord Vishnu containing his one thousand names.
Devotees believe that listening to the Vishnu Sahastranama Stotram leads to Moksha, i.e. freedom from the cycle of life. Staunch followers fast on the day of Bhishma Ekadasi. In the Hindu religion Ekadasi has come to signify a day to fasting. The auspicious Upavaas (fasting) of Bhishma Ekadasi is important as it is dedicated to Lord Vishnu. Devotees abstain from food completely on this day.
It is widely held that the demon Mura found a place for dwelling in a grain of rice, so that Lord Vishnu had to appear in the form of Ekadasi to exterminate him. That is the reason followers fast on the day of Ekadasi, and avoid grains completely. Those who are unable to observe complete fasting, keep partial fasting.
However, on Bhishma Ekadasi devotees may consume dried fruits, milk products, vegetable and nuts. But pulses and grains are completely avoided. The Upavaas is meant purify and cleanse the mind and body.