Vaikunda Ekadasi is a festival celebrated to for lord Vishnu in the month of Margazhi that falls between December and January. This festival is celebrated in a grand manner in Tirupati Balaji Temple and at Srirangam Sri Ranganatha Temple.
According to the Purana, lord Vishnu took the form of Ekadasi, female energy for killing the demon Muran which happened in Margazhi. According to popular believes, those who worship the lord on this auspicious day are assured to reach Vaikundam, heaven.
Similar to all Ekadasi days, devotees fast and observe vigil throughout the night. A few of them also medidate and sing Hari Kirtan. On these days, they do not consume rice as the demon Mura is believed to dwel in rice we eat on the day.