Sankatahara Chaturthi in August 2012 is known as Maha Sankashti or Maha Sankata Nivarana Chaturthi. Sankata Chavithi that falls before a fortnight (15 days) of Ganesh Chaturthi is very auspicious to worship Lord Ganesha. Maha Sankashti 2012 date is August 5. It falls on Bhadrapad Krishna Chaturthi in North Indian Hindi calendars whereas in Shravana masa Krishna Paksha in Marathi, Gujarati, Kannada, and Telugu Panchangam. The actual Maha Sankashti falls in September 2012 (September 4 as Angaraki Sankashti Chaturthi).
In August 2012, Sankata Chavithi starts on 5th August just after Sunrise (5:55 am) and ends on 6 August at 6.13 am. It is very auspicious that Chaturthi starts at the Sunrise and ends after Sunrise on the next day. After 15 days of this Sankashti, Ganesh Chaturthi or Vinayaka Chavithi is celebrated.
Sankashti August 2012 Moonrise timings for all cities in India is given here.