Paush Krishna Paksha Ekadashi is known as Saphala Ekadashi. It should be noted that in Hindu calendar followed in Gujarat, Maharashtra, Karnataka and Andhra Pradesh it is Margashirsha Krishna Paksha during Saphala Ekadashi.
In 2013 date of Saphala Ekadashi is 8th January. For Vaishnava Sampradaya and followers of ISKCON, Saphala Ekadashi is on the same day. For U.S. Cities Saphala Ekadashi would be on 7th January. It is interesting to note that festivals might fall one day before for U.S. location although clock runs approximately 12 hours behind to that of India.
The Parana, i.e. breaking the Ekadashi fast, for most Indian cities would be between sunrise and 9 a.m. (approximately).
In 2013 there would be two Saphala Ekadashi and the other one would fall on 28th December.