Shani Pradosh vrat,also known as Shani Pradosham, is observed on Trayodsai tithi which falls on Sanivar or Saturday in any lunar fortnight (Shukla Paksha or Krishna Paksha / Sud paksh or Vad paksh) in a Hindu month. In August 2010, Sani Trayodasi or Sani Pradosh date is August 7 and August 21. Shani Pradosh is most auspicious to worship Lord Shiva. Time to perform Shani Pradosh puja on August 7 and August 21, 2010, is 4.3o to 6 in the evening.
Sani pradosh vratha katha is associated with the legend of King Chandrasen of Ujjain. The story of Shani pradosh is mentioned in ‘Guru Charitra’ as conversation between Shri SriPad Srivallabha and a woman. He told the woman that Shani prodosh vrat is a most powerful vrata and if dutifully observed, it will endow you a son like Lord Krishna himself.