From the very beginning of His childhood life Sri Caitanya Mahaprabhu introduced the system of observing a fast on the Ekadasi day. In the Bhakti-sandarbha, by Srila Jiva Goswami, there is a quotation from the Skanda Puran admonishing that a person who eats grains on Ekadasi becomes a murderer of his mother, father, brother and spiritual master, and even if he is elevated to a Vaikuntha planet, he falls down. On Ekadasi, everything is cooked for Vishnu, including regular grains and dhal, but it is enjoined that a Vaisnava should not even take visnu-prasadam on Ekadasi. It is said that a Vaisnava does not accept anything eatable that is not offered to Lord Visnu, but on Ekadasi a Vaisnava should not touch even maha-prasadam offered to Visnu, although such prasadam may be kept for being eaten the next day. It is strictly forbidden for one to accept any kind of grain on Ekadasi, even if it is offered to Lord Visnu. = REF. Adi 15.10 Ekadasi literally means “the eleventh day.” There are two Ekadasis in a month : on the eleventh day after the new moon, and on the eleventh day after the full moon.
Of all vratas (vows of austerity), Visnu is most pleased with those who observe the Ekadasi vrata. Therefore anyone who does not observe the vrata and eats grains on Ekadasi is said to eat sin, for he disobeys the Lord’s order and thus displeases Him. Thus, although Ekadasi fasting is good for health, it should be undertaken principally for the sake of pleasing Krsna. To observe Ekadasi vrata and Dvadasi vrata means to please the Supreme Personality of Godhead. Those interested in advancing in Krsna consciousness must observe Ekadasi regularly.(SB 9.4.29) Hari Bhakti Vilasa quotes from many scriptures to establish the necessity of observing the Ekadasi vrata, to glorify the vrata, to establish the proper day on which to observe the vrata, and to give the proper mode of observance. Observing the Ekadasi vrata (vow of austerity) is one of the principal duties of Vaisnavas. To fail to do so is a serious offence