In devotional service there are certain activities which are called determined, such as fasting on certain days, like the eleventh day of the moon, Ekādaśī, and on the appearance day of the Lord. All these rules and regulations are offered by the great ācāryas for those who are actually interested in getting admission into the association of the Supreme Personality of Godhead in the transcendental world.
(Bhagavad-gita-----9:14-----purport).

Sometimes it is very troublesome to rise early in the morning, but whatever voluntary trouble one may suffer in this way is called penance. Similarly, there are prescriptions for fasting on certain days of the month. One may not be inclined to practice such fasting, but because of his determination to make advancement in the science of Kṛṣṇa consciousness, he should accept such bodily troubles when they are recommended.
(Bhagavad-gita-----10:4-5-----purport).

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).

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. By following such a process one becomes completely detached from all material activities.
(Bhagavad-gita-----14:27-----purport).

There are persons who manufacture modes of austerity and penance which are not mentioned in the scriptural injunctions. For instance, fasting for some ulterior purpose, such as to promote a purely political end, is not mentioned in the scriptural directions. The scriptures recommend fasting for spiritual advancement, not for some political end or social purpose. Persons who take to such austerities are, according to Bhagavad-gītā, certainly demoniac. Their acts are against the scriptural injunctions and are not beneficial for the people in general. Actually, they act out of pride, false ego, lust and attachment for material enjoyment. By such activities, not only is the combination of material elements of which the body is constructed disturbed, but also the Supreme Personality of Godhead Himself living within the body. Such unauthorized fasting or austerities for some political end are certainly very disturbing to others. They are not mentioned in the Vedic literature. A demoniac person may think that he can force his enemy or other parties to comply with his desire by this method, but sometimes one dies by such fasting. These acts are not approved by the Supreme Personality of Godhead, and He says that those who engage in them are demons. Such demonstrations are insults to the Supreme Personality of Godhead because they are enacted in disobedience to the Vedic scriptural injunctions.
(Bhagavad-gita-----17:5-6-----purport).

Drinking water only, without solid food, is also considered fasting. This is necessary for advancement of spiritual knowledge. A foolish man wants to be a cheap yogī without observing the regulative principles. A man who has no control over the tongue at first can hardly become a yogī. Yogī and bhogī are two opposite terms. The bhogī, or the merry man who eats and drinks, cannot be a yogī, for a yogī is never allowed to eat and drink unrestrictedly. We may note with profit how Dhṛtarāṣṭra began his yoga system by drinking water only and sitting calmly in a place with a spiritual atmosphere, deeply absorbed in the thoughts of the Lord Hari, the Personality of Godhead.
(Srimad Bhagavatam-----1:13:53-----purport).

Starvation can be mitigated by drinking water. It is sometimes found that if a person who has taken a vow to fast takes a little water at intervals, the exhaustion offasting is at once mitigated. In the Vedas it is also stated, āpomayaḥ prāṇaḥ: "Life depends on water."
(Srimad Bhagavatam-----3:26:43-----purport).

Śrīla Viśvanātha Cakravartī Ṭhākura has suggested how one can conquer lusty desires for sense gratification. One cannot give up thinking of women, for thinking in this way is natural; even while walking on the street, one will see so many women. However, if one is determined not to live with a woman, even while seeing a woman he will not become lusty. If one is determined not to have sex, he can automatically conquer lusty desires. The example given in this regard is that even if one is hungry, if on a particular day he is determined to observe fasting, he can naturally conquer the disturbances of hunger and thirst.
(Srimad Bhagavatam-----7:15:22-----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).

Lord Caitanya also explained the symptoms of a holy man and the process of satisfying the sage and rejecting the society of undesirable persons. The Lord also advised that one should hear Śrīmad-Bhāgavatam constantly. Also to be followed are duties of the day and duties of the fortnight, as well as fasting on Ekādaśī day. One must also follow the duties of the month and observe ceremonies like the birthday of the Lord, the three specific dates of fasting Ekādaśī, Janmāṣṭamī, Vāmanadvādaśī, Śrī Rāmanavamī and Nṛsiṁhacaturdaśī. When the fast days overlap with other days (viddhā), they are helpful in the advancement of devotional service.
(Teachings of Lord Caitanya).