The story of Uthpanna Ekadashi or Mrugasirsa Ekadashi
In the city of Candravati, there lived a demon Murasura. He was all powerful in the Satya Yuga and was fearlessly taking control of the demi-gods, Lord Brahma, Indra — the king of heaven, the vasus, Agni and terrified them beyond imagination.
Indra, as always, the most restless king who cannot be without his throne, went to Lord Shiva and seeked his refuge. Lord Shiva directed him to the rider of garuda vahana, the master and protector of the universe.
Indra took the advice of Lord Shiva and went to his Supreme self sleeping on Aadisesha and spoke thus — “You are Siva, Brahma, and also Vishnu, the maintainer of the three worlds. You are the gods of the sun, moon, and fire. You are the clarified butter, the oblation, the sacred fire, the mantras, the rituals, the priests, and the silent chanting of japa. You are the sacrifice itself, its sponsor, and the one who enjoys the result, the Supreme Personality of Godhead. Nothing within these three worlds, whether movable or immovable, can exist independent of You! Supreme Lord, Lord of lords, You are the protector of those who take shelter of You. Oh supreme mystic, Oh shelter of the fearful, please rescue and protect us. We demigods have been defeated by the demons and have thus fallen from the heavenly realm. Deprived of our positions, Oh Lord of the universe, we are now wandering about this earthly planet.”
Supreme Personality of Godhead, replied, “What demon possesses such great powers of delusion that he has been able to defeat all the demigods? What is his name, and where does he live? Where does he get his strength and shelter? Tell me everything, Oh Indra, and do not fear.”
Indra obliged — “Mura, the son of Nadijangha who was once a powerful demon of the Brahman dynasty. He lives in Chandravati city. He has got all the demi-gods under his control and has taken my role as the Lord of Heaveans. Light emanates from him, rain pours at his wish, wind sways for his whims and fancies. The demi-gods have lost their normal life and are extremely scared of Murasura.”
Hearing the words of Indra, Lord Janardhana summoned all the demi-gods to march along with him to Chandravati City. All the demi-gods assembled to attack the asura sainya along with Mura as their chief. His Supreme Personality pierced the entire clan’s limbs and hands within seconds and at last faced Mura. Mura had supreme mystic powers and rendered defeat like blows to Lord Hrishikesha. The battle went on together for years and Mura did not get any tired. Lord of the universe felt drained on the other hand physically alone. His emotional strength was still intact and he fled the scene to relax in the Badarikasrama caves. To trick the wicked-minded Mura, Lord manifested himself as a young girl while his mortal body was lying down and sleeping inside the cave. Mura, seeing Keshava unarmed and in that peaceful posture collected all his strength to give one final blow to the Lord. The young girl, Maha Shakthi Jyothi appeared suddenly and stopped Mura with all her might. Taken aback to see such a fierce reaction from such a young girl, his concentration and will to fight got distracted. In that moment, he could see her as the thunderbolt approaching him with all her fury. She disarmed him in one glance and in rage when tried running towards her with bare hands, she slayed his head off. Lord Hari wakes up from his trance and inquired the young girl about the whole situation.

Maha Shakthi Jyothi explains — “It is I who have killed this demon after appearing from Your transcendental body. Indeed, Oh Lord Hari, when he saw you sleeping he wanted to kill You. Understanding the intention of this thorn in the side of the three worlds, I killed the evil rascal and thus freed all the demigods from fear. I am your great maha-shakti, Your internal potency, who strikes fear into the hearts of all Your enemies. I have killed this universally terrifying demon to protect the three worlds. Please tell me why You are surprised to see that this demon has been killed, Oh Lord.”
Pleased by her action Lord Janardhana grants her a wish. The maiden requests that those who fast on that very day every year will become devoid of sins and reach the lotus feet of the Supreme Being. She also defines the meaning of fasting as follows :
“I wish that half the pious credit obtained by one who fasts will accrue to one who eats only in the evening [abstaining from grains and beans], and that half of this pious credit will be earned by one who eats only at midday. Also, may one who strictly observes a complete fast on my appearance day, with controlled senses, go to the abode of Lord Vishnu for one billion kalpas after he has enjoyed all kinds of pleasures in this world. This is the boon I desire to attain by Your mercy, my Lord.”
This is how the Uthpanna Ekadashi (11th day of waning moon) started just before the start of winter as per the Brahmanda Purana. Subsequently, every ekadashi has a story attached to it explaining the significance on what happens in our body during such fasting. The changes is observed in our mind too while following the special rules of every ekadashi. The bio chemical alteration that happens in our body during Ekadasi fasting is the subject of several research papers for the past decade. This year it falls on 30/11/2021.
As a fun fact it is noted that Badrikasrama is our famous Badrinath in the present world. Have a wonderful day :-)