Vaikuntha Ekadasi or more precisely, Amrita Ekadasi is the day when the gates of the house of the legendary deva King Vishnu was opened for the annual feast to end the winter solstice celebrations that lasted for 12 days and also celebrate the victory over the Asuras after the Deva kings gained miraculous healing and cleansing powers after drinking the Amrita or nectar made from churning of the foam of the ocean during Samudra Manthana.
At the beginning of time the devas and asuras fought among themselves for domination. The Asuras, led by Bali, attacked the devas, killing many of them and gradually seizing control of the kingdoms. Indra and the other deva kings rushed to Vishnu for help, who in turn advised them to seek alliance with the Asuras to churn the magical ocean together to extract the nectar from the ocean through the Kshirasagara Manthana that will give them strength.
The deva kings agreed only because Vishnu told them that he would make sure they would only get the nectar and regain their strength and power to defeat the asuras. Both the devas and asuras proceeded to beat the ocean of nectar. All kinds of herbs were thrown into it. The task was so arduous that Vishnu had devise many ways to aid in this process and prevent it from going anywhere and also take the help of his separated wife Laxmi, after having been separated for many decades.
The ocean shake initially produced a deadly poison that Shiva drank unknowingly, changing the color of his neck to blue because of its poisonous content.
As the asura saw the nectar they rushed upon it and stole the pot before devas could intervene. Filled with greed and pride, the demons began arguing over who of them would drink the nectar in the first place, and grabbed the pot from each other. Seeing how busy they were fighting each other, Vishnu did not miss the opportunity to deceive them and fooled them by sending Mohini, a beautiful woman who fascinated the asuras and as the asuras were ecstatic and paralyzed, Vishnu remove the pot and gave it to the devas.
As soon as the devas drank it, they got strengthened with energy and defeated the asuras. Then they accepted Vishnu as the king, who again took up the position in the royal palace.