Once, when Lord Hanuman was offering prayers to his Lord Rama near the Ram setu, Shani Dev crossed by. Lord Hanuman, immersed in his great devotion, didn t see him coming. Irritated with this attitude of Hanuman ji, Shani Dev challenged him to a fight. On several refusals, when Shani Dev didn t listen and held Hanumanji s hands forcefully, Hanuman ji tied him in his tail tightly. Then, he started brushing his tail up & down against the stones of the bridge. Shani Dev started bleeding. 

When the pain became unbearable, Shani Dev requested Hanuman to release him. Hanumna released him on the condition that Shani Dev would not cause any malefic effects to Lord Rama s followers & His followers too. And, then gave him the mustard oil to apply on his wound to get relief from the pain. Since then, it is customary to offer oil to Shri Shaneshwara to soothen his pain. On offering, Shani Dev gets satisfied and bless his devotees.

 Om Shan Shanishcharaye namah!