The importance of Ekadasi
Lord Caitanya said: “To not observe complete fasting on Ekādaśī will greatly endanger one's spiritual life. Honour prasāda on Ekādaśī only by offering it obeisances and eating it the next day. In this way, one can easily cross this material ocean."
"All Vaiṣṇavas, beloved associates of the Supreme Lord, are very pleased when Ekādaśī is properly observed by simply relishing the nectar of Lord Kṛṣṇa's holy name. One should not taste anything nor should any mundane topics be discussed on Ekādaśī. All physical pleasures should be avoided. It is a Vaiṣṇava's duty to daily honour and eat only prasāda, for he never consumes unoffered food. On Ekādaśī he observes complete fasting, and the next day he breaks his fast with mahā-prasāda."
"If for some reason, one has to eat on Ekādaśī, then let the Vaiṣṇavas take anukalpa, a light meal consisting of merely fruits, roots and milk, without grains or beans or other prohibited vegetables."
“The non-Vaiṣṇavas indulge without control, offering only lame excuses that they are honouring mahā-prasāda. They do this on Ekādaśī, because they are engrossed day and night in sense enjoyment. They enjoy grains and rice on Ekādaśī and thereby eat the sins that enter grains on such days, disregarding the sanctity of ekādaśī-vrata. You must diligently execute devotional service and respect the process of devotion. You will then surely receive the blessings of Bhakti-devi Herself. Avoid the association of non-devotees and properly observe the ekādaśī-vrata by constantly chanting the Holy Name."
[Prema-Vivarta Chapter 18 - Śrī Ekādaśī]