When Lord Ram was constructing the bridge to Lanka to cross the ocean with his army, Lord Hanuman had the liability to look after the bridge in case the enemies damage it. One day, when Lord Hanuman was praying to Lord Ram sitting under a tree, Shani Dev arrived there and said that he is the most powerful God. He added that he had heard Lord Hanumaan to be very powerful, so he wanted to test his might with Lord Hanuman. He asked Lord Hanuman to open his eyes and fight with him.
In reply, Lord Hanuman opened his eyes and warned Shani Dev not to disturb him when he is meditating on his Lord and leave him alone. But, Shani Dev was resolute to pick a fight. Hence, Lord Hanuman extended his tail and bind it around Shani Dev. He tightened his tail and Shani Dev felt the stiff hold around him. Shani Dev tried get free from that tight hold, but he was not able to. Lord Hanuman waved his tail up and down that was hitting Shani Dev on the stones. He started bleeding.
When the pain became intolerable, Shani Dev requested Lord Hanumaan to release him. He also added that he would never do a same mistake again. Hanuman asked Shani Dev to promise him that he shall never cause problems to Ram’s devotees. Shani Dev promised that he will do the same. He would never hinder in the destiny of Lord Ram’s and Hanumaan’s devotees. Hanuman then loosened his tail’s hold and freed Shani Dev.
After Shani Dev became free, he requested Lord Hanuman to give him some oil to ease his miserable pain. Lord Hanuman offered him some mustard oil. After that oil massage on his body, the pain was calmed instantly. Since then, it is traditional to offer mustard oil to Shani Dev. It soothes his body pain. He feels pleased and this makes him kind towards the Lord Ram’s devotees.
There is a spiritual analysis of the relation amid Lord Hanuman and Shani Dev.
Lord Hanuman is a symbol of selflessness, whereas Shani Dev is a symbol of ego and pride. Hence, to contradict the Karm born out of selfishness, one must be kind and unselfish like Lord Hanuman.This is true for those people who experience the evil effects of Saadhe Saatee, which is a seven and half year’s period when Shani (i.e. Saturn) is believed to trouble the three signs. One is in which the Moon is located in the natal chart of that person and the other two houses around it.