Lord Vishnu is said to be the creator of the world and the manager of destiny. He can be seen sitting on the many-headed snake who is said to be the ‘Kalswarup Sheshnag –The king of snakes’. He has a smiling and calm face with a lot of intelligence in his eyes. He is also said to be the kindest God, who immediately fulfills the wishes of his ardent devotees. Keeping in mind his kindness and blessings on us, here is the day on which we should for sure pray Lord Vishnu. The effects will be immediate and fruitful. That day is none other than Devshayani Ekadashi. The day he wakes up is known as Prabodhini Ekadashi
Benefits of Devashayani Ekadashi is mentioned by Lord Krishna to Yudhishthira in Padma Purana, Uttarakhand, Chapter 53.
2-8. I shall tell you (about that Ekadasl) named SayanI which is very auspicious, which gives heaven and salvation, which is great and which removes all sins, merely hearing about which a man would obtain the fruit of a Vajapeya (sacrifice). I have told (you) the truth and the truth (only). Nothing is greater than this for men. O lord of kings, the best one is created by the Creator for the destruction of the sins of sinners. There is no greater (vow) than this which would give salvation. Listen, O king, for this reason that the best position would be obtained by men, the listeners, even by listening to it. Those are the Vaisnavas (i.e. Visnu's devotees) who are greatly devoted to me. In Asadha, the great god Vamana (i.e. Visnu) is worshipped. He who has worshipped with lotuses the lotus-eyed Vamana in the bright half of Asadha on the day of Kamika, has honoured the entire world and has worshipped the three ancient gods (viz. Brahma, Visnu, and Siva); so also, he who has observed the Ekadasl—the day of (i.e. sacred to) Visnu—has honoured the entire world and worshipped the three ancient gods.
27-37. A man who observes the vow goes to the best posi- tion. For this reason, O king, it should be carefully observed.
Rituals are also described in brief in the same chapter.
On that day on which the lord of gods holding a conch, a disc, and a mace sleeps, a man should worship the god holding a conch, a disc, and a mace, and he should especi- ally, with devotion, keep awake at night. (Even) Brahma is not able to measure the religious merit of (i.e. due to) it. O king, he who, in this way, observes this best vow of EkadasI, which removes all sins, gives pleasures and salvation, even though a candala, always lives in my heaven doing what is dear to me.
What are the Rituals and Benefits od Dev-Shayani Ekadashi ?
The benefits of keeping ekdashi is found in Padma-Purana Part – V , Chapter 15 – The Importance of Ekadashi , where Shaunaka asks Suta about “Parasvaparivartana”.
2.11 : Suta Said - : A man should observed fast on the auspicious (day of ) Ekadashi. Which is best of all vows , should keep awake and highly decorate Vishnu.
O brahmana the man who worships Vishnu with Tulsi leaves , obtain the fruits of a core of sacrifices. O brahmana , he who would offer a lamp full of ghee (to Vishnu) on the Vishnu day in the end goes to Vishnu’s place.
Do not bring to me these meritorious being , who observe fast on (the day of ) Vishnu’s going to bed or his turning aside
That fruit is obtained by him who fasts on an (Ekadasi-) day which is obtained by bathing in Ganga etc. and at holy places or at the (time of) lunar or solar eclipse..
He having worshipped (i.e. who worships) with lotus-garlands lord Visnu after having properly broken the fast, does not have a place in mother's womb (i.e. is not reborn).
There is no greater means than Ekadasi for (going to) the next world. He who, full of many sins, observes (a fast on) the day of Visnu, is freed from all sins and goes to Visnu's abode.
The rituals are not much discussed in that chapter but from the description we come to know about these things.