Vaikuntha Ekadashi is a Hindu occasion and festival. It is primarily observed by Vaishnavas, who regard it to be a special ekadashi. It coincides with Mokshada Ekadashi or Putrada Ekadashi. It is observed on the 11th lunar day of the waxing lunar fortnight of the solar month of Dhanu. This falls between 16 December and 13 January in the Gregorian calendar. From Wikipedia
