This year, Vaikunta Ekadashi is on January 8, 2017.
The significance of Vaikunta Ekadashi is mentioned in the Padma Purana. The legend says that the Devas were unable to bear the tyranny of ‘Muran’, a demon, approached Shiva, who directed them to Vishnu. A battle ensued between Vishnu and the demon and Vishnu realized that a new weapon was needed to slay Muran. In order to take rest and create a new weapon, Vishnu retired to a cave named for the goddess Haimavati in Bhadrikashrama. When Muran tried to slay Vishnu, who was sleeping, the female power that emerged from Vishnu burned Muran to ashes with her glance. Vishnu, who was pleased, named the goddess ‘Ekadashi’ and asked her to claim a boon. Ekadashi, instead, beseeched Vishnu that people who observed a fast on that day should be redeemed of their sins. Vishnu thus declared that people who observed a fast on that day and worshipped Ekadashi, would attain Vaikunta. Thus came into being the first Ekadashi, which was a Dhanurmasa Shukla Paksha Ekadashi.
Vishnu opened the gate of Vaikuntam (his abode) for two demons in spite of their being against him. They also asked for the boon that whoever listens to their story and sees the image of Vishnu coming out of the door (called Dwar), called Vaikunt Dwar, will reach Vaikunth as well. Temples all over India make a door kind of structure on this day for devotees to walk through.
When observed, it bestows liberation from the cycle of birth and death.
The demon Muran stands for the Rajasic and Tamasic qualities in people, attributed to lust, passion, inertia, arrogance etc. When one conquers these tendencies, one attains the purity of mind, Satva, indispensable for attaining of moksha, the liberation or realization of the self. For realizing the self as pure awareness, purity of mind is required. Fasting helps to keep at bay tendencies which could be triggered by intake of certain foods. Keeping vigil in the night is symbolic of awareness, or being watchful of the contents of the mind. When the mind is looked at, it becomes still. To abide in the stillness is to attain freedom or peace, acquired through merging of the mind with the self. This is symbolic of the mind automatically being absorbed at the sight of Vishnu after the arduous fast and vigil.