A person in Kṛṣṇa consciousness does not eat anything which is not first offered to Kṛṣṇa. Therefore, only the Kṛṣṇa conscious person can attain perfection in yoga practice. Nor can one who artificially abstains from eating, manufacturing his own personal process of fasting, practice yoga. The Kṛṣṇa conscious person observes fasting as it is recommended in the scriptures.
(BHAGAVAD-GITA-----6:16------PURPORT).

The best thing to do on fasting days is to remember the pastimes of Govinda and to hear His holy name constantly.
(NECTAR OF DEVOTION).

After arriving in Kurukṣetra, the members of the Yadu dynasty took their baths ceremoniously, with self-control, as enjoined in the śāstras, and they observed fasting for the whole period of the eclipse in order to nullify the reactions of their sinful activities.
(KRSNA BOOK).

The real reason for observing fasting on Ekādaśī is to minimize the demands of the body and to engage our time in the service of the Lord by chanting or performing similar service.
(NECTAR OF DEVOTION).

The Govardhana-pūjā ceremony took place on the new-moon day. After this, there were torrents of rain and hailstorms imposed by King Indra for seven days. Nine days of the waxing moon having passed, on the tenth day King Indra worshiped Lord Kṛṣṇa, and thus the matter was satisfactorily settled. After this, on the eleventh day of the full moon, Ekādaśī, Mahārāja Nanda observed fasting for the whole day, and just early in the morning of the next day, Dvādaśī, he went to take a bath in the river Yamunā.
(KRSNA BOOK).

However, one should not fast unnecessarily or against Vedic injunctions. One should not fast for some political purpose; that is described in Bhagavad-gītā as fastingin ignorance, and anything done in ignorance or passion does not lead to spiritual advancement. Everything done in the mode of goodness does advance one, however, and fasting done in terms of the Vedic injunctions enriches one in spiritual knowledge.
(BHAGAVAD-GITA-----10:4-5------PURPORT).

In the Brahma-vaivarta Purāṇa it is said that one who observes fasting on Ekādaśī day is freed from all kinds of reactions to sinful activities and advances in pious life. The basic principle is not just to fast, but to increase one's faith and love for Govinda, or Kṛṣṇa.
(NECTAR OF DEVOTION).

Devotional service to the Lord is very simple: one should always engage in the service of the Lord, should eat the remnants of foodstuffs offered to the Deity, smell the flowers offered to the lotus feet of the Lord, see the places where the Lord had His transcendental pastimes, read of the different activities of the Lord, His reciprocation of love with His devotees, chant always the transcendental vibration Hare Kṛṣṇa, Hare Kṛṣṇa, Kṛṣṇa Kṛṣṇa, Hare Hare/ Hare Rāma, Hare Rāma, Rāma Rāma, Hare Hare, and observe the fasting days commemorating the appearances and disappearances of the Lord and His devotees.
(BHAGAVAD-GITA-----14:27------PURPORT).

However, one should generally accept the following principles to properly execute devotional service: (1) take shelter of a bona fide spiritual master, (2) receive initiation from the spiritual master, (3) serve the spiritual master, (4) inquire and learn love from the spiritual master, (5) follow in the footsteps of holy persons devoted to the transcendental loving service of the Lord, (6) prepare to give up all kinds of enjoyment and miseries for the satisfaction of Kṛṣṇa, (7) live in a place where Kṛṣṇa had His pastimes, (8) be satisfied by whatever is sent by Kṛṣṇa for the maintenance of the body and hanker for no more, (9) observe fasting on Ekādaśī day (this occurs on the eleventh day after the full moon and the eleventh day after the new moon. On such days no grains, cereals or beans are eaten; simply vegetables and milk are moderately taken, and chanting Hare Kṛṣṇa and reading scriptures are increased.), (10) show respect to devotees, cows and sacred trees like the banyan tree.
(TEACHINGS OF LORD CAITANYA).

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