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).

Some things which give impetus or stimulation to ecstatic love of Kṛṣṇa are His transcendental qualities, His uncommon activities, His smiling features, His apparel and garlands, His flute, His buffalo horn, His leg bells, His conchshell, His footprints, His places of pastimes (such as Vṛndāvana), His favorite plant (tulasī), His devotee and the periodical occasions for remembering Him. One such occasion for remembrance is Ekādaśī, which comes twice a month on the eleventh day of the moon, both waning and waxing. On that day all the devotees remain fasting throughout the night and continuously chant the glories of the Lord.
(Nectar of Devotion).

One can perform penance in accordance with those principles. For example, to undergo serious penances one may observe fasting on Janmāṣṭamī, the day on which Kṛṣṇa appeared, and on the two days of Ekādaśī (the eleventh day after the new moon and the eleventh day after the full moon). As far as charity is concerned, it is plain that charity should be given to the devotees of Kṛṣṇa who are engaged in His devotional service to spread the Kṛṣṇa philosophy, or Kṛṣṇa consciousness, throughout the world. Kṛṣṇa consciousness is a benediction to humanity.
(Bhagavad-Gita------11:54------purport).

Renunciation in Kṛṣṇa consciousness is so strong that it cannot be deviated by any attractive illusion. One has to perform devotional service in full tapasya, austerity. One should fast on the two Ekādaśī days, which fall on the eleventh day of the waxing and waning moon, and on the birthdays of Lord Kṛṣṇa, Lord Rāma and Caitanya Mahāprabhu. There are many such fasting days. Yogena means "by controlling the senses and mind." Yoga indriya-saṁyamaḥ. Yogena implies that one is seriously absorbed in the self and is able, by development of knowledge, to understand his constitutional position in relationship with the Superself.
(Srimad Bhagavatam------3:27:22------purport).

There are many statements about the festive days in connection with Kṛṣṇa's different activities. One of these festive days is Janmāṣṭamī, the day of Kṛṣṇa's birth. This Janmāṣṭamī day is the most opulent festival day for the devotees, and it is still observed with great pomp in every Hindu house in India. Sometimes even the devotees of other religious groups take advantage of this auspicious day and enjoy the performance of the ceremony of Janmāṣṭamī. Ecstatic love for Kṛṣṇa is also aroused on the days of Ekādaśī, which are other festive days in connection with Kṛṣṇa.
(Nectar of Devotion).

In their two houses Lord Caitanya Mahāprabhu begged food on the Ekādaśī day and personally ate it.
The injunction to fast on Ekādaśī is especially meant for devotees; on Ekādaśī there are no restrictions regarding food that may be offered to the Lord. Lord Śrī Caitanya Mahāprabhu took the food of Lord Viṣṇu in His ecstasy as viṣṇu-tattva.
(Sri Caitanya Caritamrta------1:10:71-------translation and purport).

A devotee is not concerned with enjoyment or distress; he simply desires to execute devotional service. It is said in the Śrīmad-Bhāgavatam that devotional service should be apratihatā, unchecked by the material conditions of happiness or distress. The devotee undergoes processes of austerity such as observing Ekādaśī and similar other fasting days and refraining from illicit sex life, intoxication, gambling and meat-eating. Thus he becomes purified from the reactions of his past impious life, and because he engages in devotional service, which is the most pious activity, he enjoys life without separate endeavor.
(Srimad Bhagavatam------4:12:13------purport).

The Caitanya-bhāgavata, Ādi-khaṇḍa, Chapter Six, fully describes the Lord's accepting viṣṇu-prasādam on the Ekādaśī day at the house of Jagadīśa and Hiraṇya. Regular prasādam is offered to Lord Viṣṇu on Ekādaśī because although fasting is recommended for devotees on Ekādaśī, it is not recommended for Lord Viṣṇu. Once on Ekādaśī in the house of Jagadīśa and Hiraṇya Paṇḍita there were arrangements for preparing special prasādam for Lord Viṣṇu, and Lord Caitanya Mahāprabhu asked His father to go there to ask for the viṣṇu-prasādam because He was feeling sick. The house of Jagadīśa and Hiraṇya Paṇḍita was situated about two miles from the house of Jagannātha Miśra. Therefore when Jagannātha Miśra, on the request of Śrī Caitanya Mahāprabhu, came to ask Jagadīśa and Hiraṇya for the prasādam, they were a little astonished. How could the boy understand that special prasādam was being prepared for Lord Viṣṇu? They immediately concluded that Nimāi must have supernatural mystic power. Otherwise how could He understand that they were preparing special prasādam? Therefore they immediately sent the food to Lord Caitanya Mahāprabhu through His father, Jagannātha Miśra. Nimāi was feeling sick, but immediately after eating the viṣṇu-prasādam He was cured, and He also distributed the prasādam among His playmates.
(Sri Caitanya Caritamrta------1:14:39------purport).

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.
(Teachings of Lord Caitanya).

Persons who have completely taken shelter of the lotus feet of the Lord should recite this narration of Dhruva Mahārāja without taking remuneration. Specifically, recitation is recommended on the full moon or dark moon day, on the day after Ekādaśī, on the appearance of the Śravaṇa star, at the end of a particular tithi, or the occasion of Vyatīpāta, at the end of the month, or on Sunday. Such recitation should of course be performed before a favorable audience. When recitation is performed this way, without professional motive, the reciter and audience become perfect.
(Srimad Bhagavatam------4:12:49-50------translation).

From the very beginning of His childhood life Śrī Caitanya Mahāprabhu introduced the system of observing a fast on the Ekādaśī day. In the Bhakti-sandarbha, by Śrīla Jīva Gosvāmī, there is a quotation from the Skanda Purāṇa admonishing that a person who eats grains on Ekādaśī becomes a murderer of his mother, father, brother and spiritual master, and even if he is elevated to a Vaikuṇṭha planet, he falls down. On Ekādaśī, everything is cooked for Viṣṇu, including regular grains and dhal, but it is enjoined that a Vaiṣṇava should not even take viṣṇu-prasādam on Ekādaśī. It is said that a Vaiṣṇava does not accept anything eatable that is not offered to Lord Viṣṇu, but on Ekādaśī a Vaiṣṇava should not touch even mahā-prasādam offered to Viṣṇu, although such prasādam may be kept for being eaten the next day. It is strictly forbidden for one to accept any kind of grain on Ekādaśī, even if it is offered to Lord Viṣṇu.
(Sri Caitanya Caritamrta------1:15:9------purport).

According to the Vaiṣṇava calendar, there are many fasts, such as Ekādaśī and the appearance and disappearance days of God and His devotees. All of these are meant to decrease the fat within the body so that one will not sleep more than desired and will not become inactive and lazy. Overindulgence in food will cause a man to sleep more than required. This human form of life is meant for austerity, and austerity means controlling sex, food intake, etc.
(Srimad Bhagavatam------4:28:35-36------purport).

It is a prejudice among smārta-brāhmaṇas that a widow must observe fasting on Ekādaśī but a woman who is sa-dhava—who has her husband—should not. It appears that before Lord Caitanya's request, Śacīmātā, being sa-dhava, was not observing Ekādaśī. Śrī Caitanya Mahāprabhu, however, introduced the system that a woman, even if not a widow, must observe theEkādaśī day and must not touch any kind of grains, even those offered to the Deity of Viṣṇu.
(Sri Caitanya Caritamrta------1:15:10------purport).

Śrīla Jīva Gosvāmī has given quotations from many śāstras stating that the śrāddha ceremony of oblations to the forefathers should not be performed on Ekādaśītithi. When the tithi of the death anniversary falls on the Ekādaśī day, the śrāddha ceremony should be held not on Ekādaśī but on the next day, or dvādaśī. In the Brahma-vaivarta Purāṇa it is said:
ye kurvanti mahīpāla
śrāddhaṁ caikādaśi-dine
trayas te narakaṁ yānti
dātā bhoktā ca prerakaḥ
If one performs the śrāddha ceremony of oblations to the forefathers on the Ekādaśī tithi, then the performer, the forefathers for whom the śrāddha is observed, and the purohita, or the family priest who encourages the ceremony, all go to hell.
(Srimad Bhagavatam------7:14:20-23------purport).

Betel nuts are an intoxicant, and therefore the regulative principles prohibit eating them. Śrī Caitanya Mahāprabhu's pastime of fainting after eating betel nuts is a solid instruction to all of us that one should not touch betel nuts, even those offered to Viṣṇu, just as one should not touch grains on the Ekādaśī day. Of course, Lord Caitanya Mahāprabhu's fainting had a particular purpose. As the Supreme Personality of Godhead, He can do whatever He likes and eat whatever He wants, but we should not imitate His pastimes.
(Sri Caitanya Caritamrta------1:15:16------purport).

Thus end the Bhaktivedanta purports of the Seventh Canto, Fifteenth Chapter, of the Śrīmad-Bhāgavatam, entitled "Instructions for Civilized Human Beings."
—Completed on the night of Vaiśākhī Śukla Ekādaśī, the tenth of May, 1976, in the temple of the Pancatattva, New Navadvipa (Honolulu), by the mercy of śrī-kṛṣṇa-caitanya prabhu nityānanda śrī-advaita gadādhara śrīvāsādi-gaura-bhakta-vṛnda. Thus we may happily chant Hare Kṛṣṇa, Hare Kṛṣṇa, Kṛṣṇa Kṛṣṇa, Hare Hare/ Hare Rāma, Hare Rāma, Rāma Rāma, Hare Hare.
(Srimad Bhagavatam------7:15:80------purport).

The Cāturmāsya period begins in the month of Āṣāḍha (June-July) from the day of Ekādaśī called Śayanā-ekādaśī, in the fortnight of the waxing moon. The period ends in the month of Kārtika (October-November) on the Ekādaśī day known as Utthānā-ekādaśī, in the fortnight of the waxing moon. This four-month period is known as Cāturmāsya. Some Vaiṣṇavas also observe it from the full-moon day of Āṣāḍha until the full-moon day of Kārtika. That is also a period of four months. This period, calculated by the lunar months, is called Cāturmāsya, but others also observe Cāturmāsya according to the solar month from Śrāvaṇa to Kārtika. The whole period, either lunar or solar, takes place during the rainy season. Cāturmāsya should be observed by all sections of the population.
(Sri Caitanya Caritamrta------2:4:169------purport).

As we generally perform devotional service on Ekādaśī by not eating grains, it is generally recommended that on Dvādaśī one not consume anything but milk. Payo-vrata and arcana devotional service to the Supreme Lord should be performed with a pure devotional attitude (bhaktyā). Without bhakti, one cannot worship the Supreme Personality of Godhead.
(Srimad Bhagavatam------8:16:25------purport).

Śrīla Bhaktisiddhānta Sarasvatī Ṭhākura explains those inscriptions as follows: (1) Śrī Puruṣottama Yati appeared as the instructor of many learned men. He was a very favorite devotee of Lord Viṣṇu. (2) His preaching was accepted throughout the world with great respect, and by his power he liberated many nondevotees with strong reason and logic. (3) He initiated Ānanda Tīrtha and converted many foolish men to accept sannyāsa and punished them with his rod. (4) All his writings and words are very potent. He gave people devotional service to Lord Viṣṇu so they could be elevated to liberation in the spiritual world. (5) His instructions in devotional service were able to elevate any man to the lotus feet of the Lord. (6) Narahari Tīrtha was also initiated by him and became the ruler of Kaliṅga Province. (7) Narahari Tīrtha fought with the Śabaras, who were caṇḍālas, or hunters, and thus saved the temple of Kūrma. (8) Narahari Tīrtha was a very religious and powerful king. (9) He died in the Śaka Era 1203, in the month of Vaiśākha, in the fortnight of the moon's waxing period, on the day of Ekādaśī, after the temple was constructed and dedicated to the holy name of Yogānanda Nṛsiṁhadeva. The tablet is dated 1281 A.D., 29 March, Saturday.
(Sri Caitanya Caritamrta------2:7:113------purport).

To observe Ekādaśī-vrata and Dvādaśī-vrata means to please the Supreme Personality of Godhead. Those interested in advancing in Kṛṣṇa consciousness must observe Ekādaśī-vrata regularly. Mahārāja Ambarīṣa's queen was equally as qualified as the King. Therefore it was possible for Mahārāja Ambarīṣa to engage his life in household affairs. In this regard, the word tulya-śīlayā is very significant. Unless a wife is equally as qualified as her husband, household affairs are very difficult to continue.
(Srimad Bhagavatam------9:4:29------purport).

The Dīpāvalī festival takes place on the dark-moon night in the month of Kārtika (October-November). The Rāsa-yātrā, or rāsa dancing of Kṛṣṇa, takes place on the full-moon night of the same month. Utthāna-dvādaśī takes place the day after Ekādaśī in the waxing fortnight of the moon in the same month. All the devotees of Śrī Caitanya Mahāprabhu participated in all these festivals.
(Sri Caitanya Caritamrta------2:15:36------purport).

Caitanya Mahāprabhu has very strongly recommended that one not commit any offense at the lotus feet of a devotee. Such offenses are compared to a mad elephant because when a mad elephant enters a garden it causes devastation. Therefore one should be extremely careful not to commit offenses at the lotus feet of a pure devotee. Actually Mahārāja Ambarīṣa was not at all at fault; Durvāsā Muni unnecessarily wanted to chastise him on flimsy grounds. Mahārāja Ambarīṣa wanted to complete the Ekādaśī-pāraṇa as part of devotional service to please the Supreme Personality of Godhead, and therefore he drank a little water. But although Durvāsā Muni was a great mystic brāhmaṇa, he did not know what is what. That is the difference between a pure devotee and a so-called learned scholar of Vedic knowledge.
(Srimad Bhagavatam------9:4:68------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. 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. 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).