In our Hindus society, it is habitual to bath Shani dev in mustard Oil. Many people also raised this question that why people offer oil to Shani dev. There is a short story behind this.
As per Hindu Epic Ramayana, when the bridge was constructed from Rameswaram to Lanka by Ram’s army, Hanumanji had the responsibility of looking after the security of bridge lest the enemy damage it. One day Hanumanji sat under the trees and offering prayer to God Rama. God Shanidev went there and said to Hanumanji, “I am the powerful god Shani dev. I heard you are also so powerful, so I want to test your might with me. Open your eyes and fight with me.”
In response to shanidev, Hanumanji opened his eyes & said “Now I am meditating upon my lord. I request you don’t disturb me in my prayer & leave me alone.” But God Shani dev was stable on picking a fight. At the same time, hanumanji stretched out his tail and started binding it around Shani dev. slowly he tightened the coiled tail. God Shani dev tried to free himself but he could not. Hanumanji moved his tail up & down hitting him against the stone of bridge. Shanidev started bleeding.
When the pain was unbearable, God Shani requested to him, “Release me. I will never do a similar mistake again.” Hanumanji replied, “If you promise me that you shall never afflict Ram’s devotees then I will let you go” In the great pain shanidev promised to Hanumanji, “I will do as you say, I will never interfere in the destiny of those devoted to you & Ram.”
When shanidev became free, He asked to Hanuman, “Could you give me some oil to relieve my wretched pain.” Then Hanuman gave the oil to Shanidev, after the spread of oil on his body, his pain was relieved immediately.
Since then it is customary to offer oil to Shanidev. It is symbolic of soothing his pain. He feels satisfied and this makes him benevolent towards the devotee.