Whenever someone has the dasha of Shani Dev on them, they are asked to pray to Lord Hanuman to please him. Shani or Saturn, is a strict planet according to astrology. It represents rules and restrictions for karmic life of a person.
According to the tale of the epic Ramayana, Lord Hanuman and Shani Dev story goes like this. After getting Vardans from Brahma, Ravan had captured many holy spirits and Gods
Year later Hanuman visits Lanka to release Devi Sita from Ravan’s prison. After reaching there he heard Shani’s cry of help. He decided to help the imprisoned planet Lord.
Hanuman had no wife or children and only wanted to serve Lord Ram. He happily agreed to let the planet Lord reside on his head.