Vaikunta Ekadasi falls in the month of Marghazhi , ( during December - January ) every year. Vaishnavam believes that on this particular day only , the inner sanctum of of the Lotus feet of the lord opened. According to vishnu purana, fasting on Vaikunta ekadasi is equivalient to fasting on the remaining 23 Ekadashis of the Hindu year. According to Padma Puran, teh female energy of the LORD slayed down Muran 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 Ekadeshi on the day of her victory over muran would erach Vaikuntam. In Mahabharatha on this day only Lord Krishna preached Bhagwat Gita to Arjun. and it was the begining of Kurushetra war. In srirangam , special poojas are conducted to mark this ekadasi.