Shattila Ekadasi, or Shatthila Ekadashi, is observed during the waning phase of moon in the month of January/February. The importance of Sattila Ekadasi was narrated to Yudhishtira by Lord Krishna and is found in the Bhavisyotara Purana. In 2015, the date of Shattila Ekadasi is January 16. Ekadashi Fasting is dedicated to Lord Vishnu and is observed on the 11th day of waning and waxing phase of moon in traditional Hindu calendar.
The name of the Ekadashi is Sat-Tila as it is believed that donating Til (Sesame) seeds on this day will help in redeeming sins. It is also considered highly auspicious to take bath in water mixed with til (sesame seeds) and offer oblations to dead ancestors with water mixed with sesame seeds.
It must be noted that Sat-tila Ekadasi takes place in North India during winter season and Sesame seeds is considered to be highly beneficial during the period.
All the normal rules associated with Ekadasi fasting are observed on the day. Food made of rice and grains are avoided. Some people do consume some sesame seeds on the day.
source – http://www.hindu-blog.com/2009/01/shattila-ekadashi-sat-tila-ekadasi.html