Mukkoti Ekadasi or Vaikunta Ekadasi
According to Vishnu Purana, fasting on Vaikunta Ekadasi is equivalent to fasting on the remaining 22 Ekadasis of the (Hindu) year.
Lord opened the gate of Vaikuntam (his abode) for two daemons inspite of they being against the Lord. They also asked for the boon that who ever listens to their story and see the image of Lord coming out of the door (called Dwaram) called Vaikunta Dwaram they reach Vaikuntam as well. Temples all over the India makes a door kind of structure on this day for devotees to pass through that.
According to Padma Purana, the female energy of Lord Vishnu slains demon Mura in the form of a damsel and protects `Devas'. Impressed by the act, Lord Vishnu names her as `Ekadasi' and gives her the boon that those who worship `Ekadasi' on the day of her victory over Mura would reach `Vaikunta' (His abode).
In Mahabaratha, Bhagavad Gita - the conversation between Lord Krishna and Arjuna at the beginning of Kurukshetra War is said to have occurred on this day.