Shani Pradosh Vrat or Shani Pradosham is observed on Trayodasi tithi which falls on Shanivaram or Saturday in any Paksha (Shukla Paksha or Krishna Paksha) of Hindu calendar. In December 2010, Sani Trayodasi or Sani Pradosh date is 18 December. Shani Pradosh is most auspicious vrata dedicated to Lord Shiva. Time to perform Shani Pradosh puja on 18 December 2010 is 4.30 to 6 in the evening.
‘Guru Charitra’, a popular scripture dedicated to Satguru Dattatreya and his divine incarnations, mentioned Shani Pradosha vrat katha. This story, associated with King Chandrasena of Ujjain, is explained as a conversation between Shripad Shri Vallabha swamy and a woman.