Today is ekādaśī-vrata, and I think that most of you have observed it. To observe Ekādaśī fully, like Ambarīṣa Mahārāja, is very rare. We should try, but we are influenced by this present age, Kali-yuga.
Mahārāja Ambarīṣa lived in Satya-yuga, and he was so strong that he could have fasted for one year, without sleep, standing on one leg, without even drinking water or breathing air. In Kali-yuga we are neither strong nor advanced; but we are fortunate. Śrī Caitanya Mahāprabhu has explained that on Ekādaśī if we are hungry, we may take some fruit, juice, or milk. We should not eat continually. It is not that we drink a liter of one type of juice, and half an hour later another type of juice, and again within half an hour we take boiled potatoes and other vegetables, then after half an hour a large drink of water, a pound of paneer with sugar, and another half liter of rabrī. We should eat only as much as is needed to allay sleep and idleness.
Remaining awake day and night, we should spend Ekādaśī chanting and remembering Śrī Kṛṣṇa, as Ambarīṣa Mahārāja did. He was the emperor of the whole world, a householder, and he was externally entangled in material attachments. Durvāsā Muni was a brāhmaṇa and a realized yogī. He could curse someone to die, and also bring a dead man back to life. He was a brahmarṣi, a brahmavādī with great powers. He thought: “I am born in a brāhmiṇical family and am in the dynasty of Śaṅkara (Śiva). I am a manifestation of Śaṅkara, the son of Atri and Anasūyā, who are highly learned uttama-bhakta Vaiṣṇavas. My parents gave so many teachings to Sītā-devī, who is Laksmī herself, when She and Rāma visited my parents’ cottage. I am the greatest and most powerful man in this world; I can do anything within a moment.” However, Durvāsā Muni never observed Ekādaśī.
Once, in his palace in Madhuvana, Mathurā, Ambarīṣa Mahārāja observed Mahā-dvādaśī, neither drinking water nor sleeping. He absorbed himself throughout the day and night in chanting, remembering, serving the Deities, and praying. The next day, after worshiping Śrī Kṛṣṇa, when he was almost ready to break his fast, Durvāsā Muni arrived at his palace.
Durvāsā was like a god, a realized brahmavādī, but he had not observed Ekādaśī. Ambarīṣa Mahārāja requested him to break Ekādaśī by taking mahā-dvādaśī-pāraṇa, after which he himself would honour prasādam. Durvāsā Muni replied, “Yes, but first I will take bath nearby in the Yamunā River. I won’t be very long.”
He delayed for such a long time, however, that only a few minutes were left before the auspicious time for breaking Ekādaśī would be over. Ambarīṣa Mahārāja wondered what to do. He thought, “A brāhmaṇa-ṛṣi, a siddha mahā-yogī, has come to my door. He is not an ordinary person. I should respect him by offering him mahā-prasādam before I myself take it. Otherwise, I will be neglecting Indian culture, and I may possibly go to hell. On the other hand, I must observe my ekādaśī-vrata, which is not a worldly activity. It is transcendental, and if I fail to observe it by not taking mahā-prasādam at the proper time, I may lose my bhakti. Should I honour this brāhmaṇa, or honour bhakti?”
After careful consideration, he chose to honour bhakti. “I cannot act against bhakti in any way, even if I lose some material prosperity or go to hell. I will not disrespect Ekādaśī; I will break it within the prescribed time.” He then drank a drop of water that had been used in the worship of Śrī Kṛṣṇa, called caraṇāmṛta.
If one observes Ekādaśī without taking juice or water, he can break the fast by taking caraṇāmṛta. But if water, fruit, juice, milk, or vegetables have been taken, one will have to break it by taking some grain; otherwise his Ekādaśī is not complete.
Ambarīṣa Mahārāja thought, “I have not taken even water during my observance of Ekādaśī, so I can take caraṇāmṛta. In this way, because I will not have taken any grains, I will also be respecting the brāhmaṇa.”
After taking a drop of caraṇāmṛta, Durvāsā Muni returned. “I know you, He shouted. “You are only a so-called bhakta. I am a siddha (perfect mystic) brāhmaṇa. You know my parentage. You know of my great powers, and yet you have dared to dishonour me. I will immediately burn you to ashes.”
He took one dreadlock from his hair, dashed it to the ground, and ordered it to become a burning fire. The fire demon then ran towards Ambarīṣa Mahārāja in an attempt to burn him to ashes. At that moment the Sudarśana cakra appeared there. Śrī Kṛṣṇa had ordered His cakra to always protect the Vaiṣṇavas, persons who have taken shelter of Him, so Sudarśana burned to ashes the fiery demon and then moved toward Durvāsā Ṛṣi, who fled, ‘taking his legs on his head.’ Durvāsā Ṛṣi then cried out, “Save me! Save me! Save me!”
When the fire demon had approached Ambarīṣa Mahārāja, the king chanted and was fearless. He thought, “Whether I live or die depends on Śrī Kṛṣṇa.” He depended fully on Śrī Kṛṣṇa, and Sudarśana cakra saved him. Durvāsā Muni, however, though a powerful brāhmaṇa and realized yogī, feared death and fled. He tried first to take shelter of his father, Lord Śiva; but seeing him approach, Lord Śiva called out to him, “Don’t come here. Go from here! I know why you are running. You are in this predicament because you have dishonoured a Vaiṣṇava. I know how influential Vaiṣṇavas are and I fear offending them. If I give you shelter, Sudarśana will also attack me.”
Durvāsā Muni then went to Lord Brahmā, who also refused to help him. After that he approached Lord Viṣṇu and prayed, “O Viṣṇu! O Viṣṇu! Save me from Your weapon. I am taking shelter of You.”
Lord Viṣṇu said, “I am powerless to protect you. I have no freedom. I am bound to My bhaktas. They have given their hearts to Me, and I have given Mine to them. I am in no way independent of My devotees.”

śrī-bhagavān uvāca
ahaṁ bhakta-parādhīno
hy asvatantra iva dvija
sādhubhir grasta-hṛdayo
bhaktair bhakta-jana-priyaḥ

(Śrīmad-Bhāgavatam 9.4.63)

The Supreme Personality of Godhead said to the brāhmaṇa: “I am completely under the control of My devotees. Indeed, I am not at all independent. Because My devotees are completely devoid of material desires, I sit only within the cores of their hearts. What to speak of My devotee, even those who are devotees of My devotee are very dear to Me.
If someone gets a thorn in his foot, that thorn cannot be removed from his head. It must be removed from his foot. So Lord Viṣṇu is saying, “The Vaiṣṇavas are everything to Me. It is My vow to protect them. If you want to be saved, you must go at once to Ambarīṣa Mahārāja. Do not see him as a family man or an ordinary kṣatriya king. Do no think ‘Oh, he is nothing.’ He is My heart. Offer him your obeisances.”
Worldly persons tend to think, “I am so learned, rich, and powerful. These bogus Vaiṣṇavas are useless and only chant Hare Kṛṣṇa, doing nothing of value.” Do not behave like this. Rather, become an actual, sincere Vaiṣṇava. Then, Śrī Kṛṣṇa will protect you and bestow upon you kṛṣṇa-prema.
On the Lord’s order, Durvāsā Ṛṣi immediately returned to Mahārāja Ambarīṣa. Durvāsā begged, “Please save me from the Sudarśana cakra. His heat is burning me.” Ambarīṣa then prayed, “O Prabhu, O Lord, if I have ever performed devotional service to Śrī Kṛṣṇa, then please save this brāhmaṇa.” No sooner were these words spoken then the Sudarśana cakra became cool and returned to Lord Viṣṇu.
Ever since Durvāsā Ṛṣi had begun fleeing from the Lord’s weapon one year earlier, Ambarīṣa Mahārāja had not eaten anything.
In fact, when he finally came before Ambarīṣa Mahārāja, the king apologized to him with great humility. He said, “I am nothing – I am not a devotee. I have neglected and dishonoured you by breaking my fast.” Durvāsā Muni fell flat at the lotus feet of Ambarīṣa Mahārāja, who embraced him.
Ambarīṣa Mahārāja asked him to be merciful and honour mahā-prasādam. Durvāsā then expressed his new realization, that bhaktas are more powerful than Kṛṣṇa Himself. Kṛṣṇa has given all of His power to the Vaiṣṇavas. But they hide this and ‘cheat’ others, presenting themselves as fallen persons. Tṛṇād api sunīcena. Although they are extremely powerful, they are also meek and humble, generous and polite.”
Durvāsā Ṛṣi gladly took mahā-prasādam, after which Ambarīṣa Mahārāja also honoured prasādam along with his whole family.
So be like that: humble, with strong faith in devotional service to Śrī Kṛṣṇa and Ekādaśī. We can give our head, life, and soul to observe Ekādaśī. Ekādaśī will protect us, for she is the mother of prema-bhakti.
Śrīla Bhaktivinoda Ṭhākura has written: mādhava-tithi, bhakti-jananī, jatane pālana kori. Ekādaśī is mādhava-tithi, Kṛṣṇa Himself. Holy days like Ekādaśī are bhakti-jananī, the mother of prema-bhakti. We should try to follow it as purely as possible, not eating and drinking many times throughout the day.
If possible, observe it without even drinking water (nirjala), or take a little prasādam once in the day if this is not possible. Many old people, who are lean and thin, can observe nirjala-ekādaśī; so why not those of you who are young and strong? Try to observe it like this. However, if by such fasting your head pains and you go to sleep, then what is the use? Try to remember Kṛṣṇa, associate with Vaiṣṇavas, read the Vedic scriptures, and offer prayers day and night. One such prayer is:

he kṛṣṇa! karuṇā-sindho
dīna-bandho! jagat-pate
gopeśa! gopikā-kānta
rādhā-kānta! namo ‘stu te

I offer my unlimited obeisances unto You, O Kṛṣṇa! You are the ocean of mercy, friend of the fallen, Lord of creation, and master of the cowherd community. You are Gopī-kānta, beloved of the gopīs, and above all You are Rādhā-kānta, the beloved of Śrīmatī Rādhikā!
Observing Ekādaśī gives even more spiritual power than brahmajñāna (realization of spirit as separate from matter) and brahmayoga-samādhi (absorption in the impersonal aspect of God).

―Śrīla Bhaktivedānta Nārāyaṇa Mahārāja