The Eleventh day of a Paksha is known as Ekadashi or Ekadasi. Ekadashi is one of the auspicious thithi for the devotees of Lord Vishnu. Hence devotees of Vishnu observe fasting during Ekadashi to please Lord Vishnu.
The Ekadashi or Ekadasi Dates for September 2012 are as folows:
Parama Ekadashi or Parama Ekadasi on 12th September 2012.
Parsava Ekadashi or Parsava Ekadasi on 26th September 2012.
Observing Ekadasi Vrat or Ekadashi Vrat is considered to be highly auspicious and also gets rid of all the sins that are done in the past.