This somvar katha is all about the Monday's fasting story, which went about sixteen weeks. People who fast are told to recite it every Monday, while performing the fast for Shivji. This story is based on the life and teachings of Shiva.
I think you can refer the link below to view the somvar katha.
According to Hindu belief fasting on Monday for 16 weeks is a must for people who worship Lord Shiva. The rituals for other fasts and Monday fasts are all the same but the story behind it will explain better about its importance. Somvaar fast is observed to please Lord Shiva and Parvathi. The story travels to northern part of India, where a rich merchant, staunch devotee of Lord Shiva fasts on every Monday for him. He didn’t have a copy of him (child) and Lord Shiva agrees to the request of Parvathi to grant him with a son but on one condition that the child will only for 12 years. Devoted merchant listens to their conversation and with a grief he brings up his son. When the child turns 11, he sends the boy to Kashi for spiritual education with his brother-in-law. Fate takes twist and turns in boy’s life due to his handsome and matured look. He ends up marrying King’s daughter in another city, replacing the physically challenged bridegroom. He reveals the truth to his wife and deserts her to complete his studies. The fateful day arrives when he dies (he completes 12years of life span). On seeing the grief and tragedy, the quality of motherhood in Parvathi springs up which makes Lord Shiva to bring back the boy to life. Then he goes back to his home along with his wife, who he deserted once.
There are many versions of somvar vrat katha, I would like to share with you one here. The story starts with Lord Shiva and Parvati visiting a town called Amaravati. In the city they found a Shiva temple and decided to spend some time there and both began playing dice game. In the meanwhile the temple priest came to them and for the sake of curiosity Parvati enquired who will win the game. The priest who was not serious about the question answered that it will be Lord Shiva who is going to win the game. But the dice game was won by Parvati. This incident irritated Parvati and she cursed the priest to become a leper for lying. The priest who felt the wrath became a leper and lived a miserable life.
One day some fairies came to earth and seeing his terrible state, told one of the fairies, ‘Take fast for 16 Mondays without a break, and on the seventeenth Monday make prasaad or offerings with flour ghee and jaggery. Distribute the sweet among your friends and family and you also eat some.’ The priest did 16 day fasting as per the words of the fairy and he recovered and lived a normal life thereafter. When one another day Lord Shiva and Parvati visited the temple again, Parvati was astounded to see the priest in his normal self. When she enquired, priest said about the fast and his story. Parvati who was pleased with the story of the priest decided to do the fasting and on the seventeenth Monday, Karthikeya her son who was not with her came to visit her. Karthikeya asked what the power that asked him to visit Parvati was and thus she mentioned the somvar vrat katha to him.
Inspired from the story Karthikeya observed the fasting for the safe return of his Brahmin friend. The Brahmin friend then did the vrat for getting a beautiful bride. He gets married to a beautiful lady and on their first night the Brahman narrated her story of fasting. Hearing about it she also fasted for 16 Mondays for a baby boy. Baby boy was born and when he grew up he fasted for getting a kingdom for his parents. He married a princess and soon after the marriage the father of the princess passed away making him the King.
The newlywed king continued his fasts and on the seventeenth Monday he arranged a big feast. The Queen refused to go with him to temple. The queen was cursed. An oracle told the King to abandon the queen or else she will bring curse to the palace. Hearing the oracle King had no other go but to abandon the queen. Thus the queen left the palace, but wherever she went the curse followed. Whatever she touched became impure-water, milk or food. Finally a temple priest seeing her plight asked her to do the fasting for 16 Mondays and on the seventeenth Monday she was called back by her husband. This was a lesson learnt and from that day every year both King and Queen did the somvaara vrat every year. They lived happy ever after and after their death they found abode in Shiva’s city, Shivapuri. It is believed that a person who observes fasts on sixteen Mondays will get all physical pleasures and enters Shivapuri after death.