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. Bhaktivinoda Thakura has sung, madhava-tithi bhakti-janane jatane palana kari (Suddha-bhakata, from Saranagati): "I observe with great care the holy days like Ekadasi and Janmashtami, for they are the mother of devotion." All people, male and female, of all castes and ages, from eight to eighty, should observe the Ekadasi vrata in all circumstances. This applies to the Ekadasis of both the waxing (gaura or sukla) and waning (krsna paksa) phases of the moon.
What is Ekadashi?
Observance of Ekadasi vrata is calculated to begin at sunrise. The period before sunrise is considered to be part of the previous day. So the Ekadasi vrata starts at sunrise and lasts until the next sunrise. We observe Ekadasi from sunrise to sunrise. Twelve midnight is the western astronomical calculation, but the Vedic astronomical calculation begins either from the sunrise or the moonrise. Generally it is sunrise. Our calculation is like this: when the sunrise is there, Ekadasi tithi (date) must be there. If the tithi begins a few minutes after the sunrise, then we accept that day as prior to Ekadasi. ( Letter to: Madhudvisa, 30 September, 1969 ) If one eats grains during the period from sunrise to sunrise, one breaks the vrata. He will not get the results of the fasting, and he will get impious reactions as well. In Vedic culture the day begins at sunrise no matter when the lunar tithi begins (The Vedic system differs from the Gregorian calendar in as much as the days are named are deemed to begin at sunrise, rather than midnight). Therefore, technically speaking, since ekadasi or any other day does not begin until sunrise, one could eat grains before sunrise. However, one should generally not eat in the pre-dawn hours, and especially not on ekadasi. Hence, ekadasi fasting and other observances should be followed from the time of waking up on the ekadasi vrata day. Some Vaisnavas are particular not to eat anything after sunset the night before, to make sure there is no undigested food left in the stomach on ekadasi day.