Utpanna Ekadasi, or Uttpatti Ekadashi, is observed during the waning phase of the moon in the month of November – December. In 2015, the date of Utpanna Ekadasi is December 7. The importance of this Ekadasi was explained to Arjuna by Lord Krishna and is found in Bhavishyottara Purana. For those devotees who like to begin the monthly Ekadasi fasting, Uttpatti Ekadasi is the Ekadashi that they should start with.
All the sins accumulated during the previous births and this birth are washed away by observing Uttpatti Ekadashi and this leads to Moksha.
Fresh beginners who want to undertake Ekadasi fasting start it with Utpanna Ekadasi. They can continue it forever or for a year. How long it is continued depends on the devotee.
All the normal rules associated with Ekadasi are observed during Utpanna Ekadashi
Source – http://www.hindu-blog.com/2008/11/utpanna-ekadashi-uttpatti-ekadasi.html