In Padmapurāṇa (Padmapurana), the glory of Ekādaśīvrata (Ekadasivrata) is hailed as far greater than all other vrata(s), and major yajña (yajna) performances.The greatness of visiting pilgrimages, donation, and other vratas – are highly praised – until there comes the issue of greatness of fasting on the Ekādaśī tithi —
tāvadgarjanti tīrthāni dānāni niyamāni ca
yāvannopoṣayejunturvāsaraṃ viṣṇuvallabham.

Hearing of this rare virtuous fruit of Ekādaśīvrata from Kṛṣṇa (Krishna), Yudhiṣṭhira (Yudhishthira) wanted to learn about the origin of Ekādaśi tithi-
utpannā sā kathaṃ deva puṇyā ekādaśi tithiḥ
kathaṃ pavitrā vuśve’smin kathaṃ vai devatāpriyā.

Kṛṣṇa started speaking that – There was a great demon called Moora in Satyayuga. He drove the gods away from svarga (the heavenly abode of the gods). The defeated gods went to seek the help of Rudra-Śiva (Siva). Mahādeva (Mahadeva)asked them to appeal to Viṣnu. Hearing everything from Indra, Viṣṇu (Vishnu) assured them that he would slay Moora and restore the heavenly abode to them. Then there broke out a great battle between the gods and the demons. All major demon warriors were slain by the Sudarśana chakra of Viṣṇu, but Moora remained untouched and undefeated. Then a wrestling bout commenced between God Viṣṇu and Moora,and it continued for a long time. But he was not able to defeat Moora. By then, all other gods had left the battlefield. Seeing this, Viṣṇu also stopped the battle and went away to Vadarikāśrama. There was a cave called Siṃhāvatī, on the mountain. God Viṣṇu was resting there. Now, following God Viṣṇu, Moora also reached the cave. Seeing Viṣṇu in a sleeping mode, he thought, even Viṣṇu has acknowledged him as an invincible warrior. So, in fear, he had left the battlefield, and come there to hide himself. Thinking all this, Moora planned to ‘kill’ God Viṣṇu inside the cave, while he was sleeping. At that moment, a beautiful maiden, decked with several weapons, emerged from the body of Viṣṇu. That maiden challenged Moora to a battle. Moora was defeated and slain by that divine maiden, emerged from the body of Viṣṇu. Waking up, Nārāyaṇa found the demon dead. Surprised, he started thinking, who made this impossible task possible? Then that maiden told Viṣṇu, ‘I found that this demon, invincible to the gods, has been ready to kill you, when you were in a reclining posture. If you were killed, the whole creation will be destroyed. Considering that, I have fought against this demon. Finally, by your grace, I have slain him, too’. This divine maiden, emerged from Viṣṇu, is known as Ekādaśī. Pleased with her prowess, Viṣṇu himself wished to grant her a boon. Ekādaśī said, “I have only one desire, if you promise that you will fulfill it, I’ll ask for the boon”. Viṣṇu took a vow that he would obviously fulfil her wish. Then she said, “Do establish me in all the three abodes, in the four yuga(s) in such a way that I can be of more virutuos power, a giver of siddhi – than all the tīrtha(s), and all other virtuous deeds or rituals. Viṣṇu said, “Let it be so. From this day, you will be renowned as Ekādaśī tithi. Those who will perform a fasting on this tithi and worship me, will attain mokṣa”.

Thus, by the grace of Viṣṇu, Ekādaśī tithi and vratas performed on that tithi became famous as more important than all other tithi , vrata and yajña(s).

There are so many vratas and tithis in Indian culture, that most of them are not possible to be performed now-a-days. But the Ekādaśī vrata is perhaps the only one vrata that is still performed. In general, Matsyapurāṇa has given instructions for observing the Ekādaśī tithi — in the morning of this tithi, one has to worship God Keśava, and perform a ritual fasting, and should remain awake at night. The next morning, after taking a bath, one should perform a homa-offering to Agni, feed the brāhmaṇas, and afterwards take food for oneself. At nigt it is advisable to sleep on the ground —
ekādaśyāṃ nirāhāraḥ samabhyarcya tu keśavam
rātrñica sakalāṃ sthitvā snānañca payasā saha.

In Agnipurāṇa, there are more elaborate instructions that in order to observe Ekādasī, one needs to practice self-control regarding food and conjugal life, from Daśmī (the previous tithi) itself. One should avoid partaking of meat, and physical consummation. The next day, on Ekādaśi, the whole day and night should be spent in fasting, and this should be followed in both Ekādaśī(s), in Śuklapakṣa and in Kṛṣṇpakṣa —
daśamyāṃ niyatāhāro māṃa-maithuna-varjitaḥ
ekādasyāṃ na bhunjīta pakṣayorubhayorapi.

There is also a custom of uttering a sentence of oath, before performing pāraṇa (end of the vrata) on Dvādaśī, after observing a fast on Ekādasī.

The Smārta scholars, citing the instruction of Purāṇa, have shown that the fast of Ekādaśī can be observed in two ways. One is the way of observing Ekādaśī by following what is not to be done. The second way is to go by those customs which are to be observed — such as, not drinking water from any container made of śankha (sankha), not consuming pork or meat of tortoise, and so on. This type of Niṣedhātmaka Ekādaśī (Nishedhatmaka Ekadasi, which is based on ‘don’t’s) is to be performed on both Śuklapakṣa and Kṛṣṇapakṣa. In the text called Nirṇayasindhu (Nirnayasindhu) by Kamalākara Bhaṭṭa, it is said that in order to perform Ekādaśī, on the day of Harivāsara, one needs to practice self-control from the previous day, that is, Daśamī.

In Nāradapurāṇa there are more detailed instructions about the observation of Ekādaśī. Rising early in the morning, one needs to clean himself, after tooth-brushing and taking a bath, prepare Pañcāmṛta, and with that, bathe the image of God Viṣṇu established in the household. Then one has to worship the Deity with several equipments, fragrant flowers smeared with sandalwood-paste. For this, one has to take an oath , saying that “I’ll perform Ekādaśī by remaining on a complete fast, and resume food-intake only on Dvādaśī, and I submit myself to the grace of the lotus-eyed God Srī Viṣṇu.”
– ekādaśyāṃ nirāhāraḥ sthitvā cāhaṃ pare’hani.
bhokṣye’haṃ puṇḍarīkākṣa śaraṇaṃ me bhavācyuta.

Nāradapurāṇa (Naradapurana)refers to Ekādaśī as the most favourite tithi of God Viṣṇu. Along with the fasting and the customs to be observed on Ekādaśī, it becomes very important to be in the vicinity of one’s own deity.

Nāradapurāṇa says, along with control or the organs, one has to sleep on the ground, near the image of Viṣṇu in the temple, worship the Deity,
devasya purataḥ kuryājjāgaraṃ niyato vratī
gītavādyaiśca nṛtyaiśca purāṇśravaṇadibhiḥ.
In Brahmapurāṇa, along with other customs to be observed on Ekādaśi, the glory of remaining awake throughout the night has been described with great esteem.

There is a legend about the Caṇḍala who was devoted to God Viṣṇu. This Caṇḍāla lived in Avantī, and at night of every Ekādaśī, went to the temple, and sang the praise of God Viṣṇu, with perfect notes of ṣaḍaja, gāndhāra, nikhāda and so on. The greatness of that Caṇḍalā has also been described throughout several chapters.

The glory of observing Ekādaśī became so important in later ages that the twenty four Ekādaśī(s) had been named with distinction. According to Padmapurāṇa, The name of Śuklā Ekādaśī of the month of Mārgaśīrṣa or Agrahāyaṇa Phālguna is Jayā, and the Kṛṣṇā Ekādaśī of Phālguna is Vijayā, but the Kṛṣṇā Ekādaśī is not named. The Kṛṣṇā Ekādaśī of the month of Pauṣa is called Saphalā, and the name of Śuklā Ekādaśī is Putradā. The Kṛṣṇā Ekādaśī of the month of Phālguna is Vijayā, and the Suklā Ekādaśī is Āmalakī. The Kṛṣṇā Ekādaśī of the month of Caitra is called Pāpamocanī, and the Suklā Ekādaśī is Kāmadā. The Kṛṣṇā Ekādaśī of Vaiśakha is Varuthinī, and the Suklā Ekādaśī is Mohinī. The Kṛṣṇā Ekādaśī of Jaiṣṭhya is Aparā, and Suklā Ekādaśī is Nirjalā. The Kṛṣṇā Ekādaśī of Āṣāḍḥa is called Yoginī, the the Śuklā Ekādaśī is Śayanī. The Kṛṣṇā Ekādaśī of Śrāvaṇa is Kāmikā, and the Śuklā Ekādaśī is Putradā. The Kṛṣṇā Ekādaśī of Bhādra is Ajā, but without mentioning the legend associated with Śuklā Ekādaśī, it is given a second name, Padmā. Elsewhere, the Śuklā Ekādaśī of Bhādra is Parivartinī. In Padmapurāṇa, the Kṛṣṇā Ekādaśī of Āśvina is Indirā,and the Śuklā Ekādaśī is Pāpānkuśā. The Kṛṣṇā Ekādaśī of Kārtika is called Ramā, and the Śuklā Ekādaśī is Pravodhinī.

Depending on this portion of Padmapurāṇa, mentioned above, scholars have named the twenty four Ekādaśī(s) From the Śuklā Ekādaśī of Caitra, the names of the Ekādaśī(s), in the order of Kṛṣṇā-Śuklā are — Kāmadā, Varuthinī, Mohinī, Aparā, Nirjalā, Yoginī, Śayanī, Kāmikā or Kāmadā, Putradā, Ajā, Parivartinī, Indirā, Pāpānkuśā, Ramā, Pravodhinī or Vodhinī, Utpatti, Mokṣadā, Saphalā, Putradā, Ṣaṭtilā, Jayā, Vijayā, Āmalakī or Āmardakī and Pāpamocinī.

Besides these names, other two Ekādaśī(s) have been famous.One of them is Śayana Ekādaśī, or the Śuklā Ekādaśī of Kārtika. On this day God Śrī Hari remains lying upon the coil of Śeṣanāga . The other is Utthāna Ekādaśī, that is the Śuklā Ekādaśī of Kārtika. He gets up from his reclining pose on this day.

ekādaśyāṃ tu śuklāyāmaṣāḍhe bhagavān hariḥ
bhujaṅga-śayane śete yadā kṣīrārṇavāntare.
ekādaśyāṃ tu śuklāyāṃ kārtike māsi keśavam
prasuptaṃ vodhayed rātrau śraddhābhaktisamanvitaḥ.

Regarding the determination of Ekādaśī, there are several discussions in Smṛti-texts. But the most important discussion is about whether Ekādaśī is crossed by Daśamī or Dvāaśī. Regarding the measurement of tithi, Ekādaśī is of two types– Sampūrṇā and Viddhā. Sampūrṇā Ekādaśī is the one, where the full cycle of 60 ghaṭikā is available beginning from the sunrise. But according to the Purāṇa-scholars, this is a kind of ideal conception, practically it is not possible that exactly at the time of sunrise, begins the Ekādaśī. So, citing the statement of Garuḍapurāṇa, Hemādri says that even if at least two minutes before the sunrise, the Ekādaśī begins, it is called Sampūrṇā Ekādaśī–
*udayāt prāk yadā viprā muhūrtadvayasaṃyutā
sampūrṇaikādaśī nāma tatraivopavasedgṛhī.
*ādityodayabelāyāḥ prāk-muhūrtadvayasaṃyutā
ekādaśī tu sampūrṇā viddhānyāā parikīrtitā.

aperforming vrata-upavāsa on Ekādaśī, that is penetrated by Daśamī, has been strictly condemned in Nāradapurāṇa and Garuḍapurāṇa. It has been said that in Māhābhārata, Gāndharī performed her vrata-related fasting on such a tainted Ekādaśī, so she had to lose all her hundred sons in the battle of Kuukṣetra–
daśamyekādaśīmiśrā gāndhāryā samupoṣitā
tasya putraśataṃ naṣṭaṃ tasmāttāṃ parivarjayet.
According to Garuḍapurāṇa, Asura̍s influence falls on Ekādaśī that is crossed by Daśamī, and God Śrī Hari̍s grace falls on Sampurṇā Ekadaśī. If, due to conflicts between several scriptures, there arise a doubt whether Daśamī, Ekādaśī and Dvādaśī fall on the same day, in that case the Ekādaśī-fasting is to be maintained on Dvādaśī and the Pāraṇa is to be done on Trayodaśī. If one kalā of that Ekādaśī, it has to be performed on Dvādaśī itself.