Ekadashi is the 11th day of every fortnight of Hindu month. Every month is divided into 2 fortnights - Shukla Paksha and Krishna Paksha. Ekadashi is the 11th day in each of these Pakshas - Ekadashi means 11 in Samskrita. Vaikunta Ekadasi, is considered to be the most important Ekadasi in "Sri Vaishnava" tradition. Vaikunta Ekadasi occurs during the month of Margashira and is an important festival dedicated to Lord Vishnu (In Solar Calendar, this occurs during the Ekadashi of Dhanurmasa Sukla Paksha). According to Vishnu Purana, observing the Ekadasi Vrata on Vaikunta Ekadasi day confers the benefit equivalent to observing all the 23 Ekadasis of the Hindu Calendar year.
It is believed that, on this day, the Vaikunta Dwaram or "The Gates of Heaven" are kept open. It is believed that, Bhagavad Gita (conversation between Lord Krishna and Arjuna) at the beginning of Kurukshetra War occurred on this day. It is believed that, "Amrita" that emerged out during "Saagara Mathana" (churning of the ocean) happened on this day. It is believed that those who die on this day are freed from the rounds of birth and death and they directly reach Vaikunta (the abode of Lord Vishnu).
According to Puranas....
The significance of Vaikunta Ekadasi can be traced back to the Padma Purana. According to this purana, the Devas unable to bear the tyranny of demon 'Muran' approached Shiva, who directed them to Vishnu. A battle ensued between Lord Vishnu and Muran. Vishnu realised that a new weapon was needed to kill Muran. In order to create a new weapon, Vishnu went to a cave called Himavati in Badrikashrama. But, Muran came to know that Vishnu is creating a new weapon to kill him. So, Muran tried to kill Vishnu when he was asleep. At that time a female energy emerged from Vishnu and burnt Muran to ashes with just one look. Vishnu named the female energy 'Ekadasi' and asked her to take a boon. Ekadasi, instead requested Vishnu, that people who observed a fast on that day should be released from their sins and Karma. From then on, this Ekadashi is observed as "Vaikunta Ekadashi".
Fasting and Vigil
During Vaikunta Ekadasi, Fasting is an important aspect of those associated with it. People fast the whole day and also keep vigil during the night. Chanting the names of Lord Vishnu, visiting Vishnu temples are important aspects of this day. Vishnu Temples all over the India makes a door kind of structure called "Vaikunta Dwar" on this day for devotees to pass through that.
No rice should be taken on Ekadashi days and this is very important. It is said that the sweat that fell down from the head of Brahma assumed the form of a demon and he asked the Lord to provide him an abode to dwell. Brahma said to the demon to go and dwell in the rice particles eaten by men on Ekadashi day and become worms in their stomach. For this reason rice is prohibited on Ekadashi.
It is important to look at the significance of this auspicious day from the spiritual angle. The demon Muran stands for the Rajasic and Tamasic qualities in people, which attributes to lust, passion, inertia, arrogance etc. When one conquers these tendencies, one attains the purity of mind, Sattva, indispensable for attaining moksha, liberation or realisation of the self. For realising the self as pure awareness, purity of mind is required. Fasting helps to keep tendencies at bay, which could be triggered by intake of certain food. Keeping vigil in the night is symbolic of awareness, or being watchful of the contents of the mind. Devotion gradually develops and love for God becomes intense. In the Gita Krishna tells Arjuna that: “Yoga is not for him who eats too much, nor who abstains to excess, nor who sleeps too much, nor to the excessively wakeful”. Yogi withdraws his senses from the particular sense objects. The senses are made to turn into or get involved into the mind. When one is fully established in these two practices, supreme control of the senses is achieved.