People offer mustard oil or sesame oil on Saturday to make Shani Dev happy. Only few people are aware about the reason behind offering mustard oil to him. Shani Dev tried to merge his powers along with Hanuman ji. He was meditating on Lord Ram and offering prayer to him. Therefore, Hanuman ji urged Shani Dev not to disturb him and leave him alone. But Shani Dev wanted to show his powers.
Seeing Shani Dev repeatedly fighting for war, Hanuman ji raised his tail and slammed him on big stone. As long as Shani Dev did not request him to leave him, Hanuman ji did so many times. Saturn dev’s body was injured from this work and he got very hurt. But Hanuman ji kept a condition to leave them that they will never bother him again. Saturn Dev accepted this.
Prior to going from there, Shani Dev requested Hanumanji to give oil to apply on his wounds. Hanuman accepted his request and after relieving oil, he got relief from the pain. Thus, on Saturdays, it was a practice to supply sarsa oil to Shani Dev. Applying oil on Shani Dev is a sign of relief from all kinds of pain. After overcoming their pain, Saturn becomes calm and generous towards their devotees.