Pradosh means the dusk of the early night or evening tide. This fast is observed in the evening at twilight time, and so it is called Pradosh Vrat. It is observed on the 13th day of each lunar fortnight. When it coincides with Monday, it is called Som Pradosh. It is practiced to propitiate Lord Shiv, in order to obtain his blessings and boons leading to fulfillment of one's cherished desired and spiritual upliftment. Those who practice it with unflinching faith and devotion are bound to possess wealth, health, happiness. It is believed that on this day all Devtaa and Devee go to Kailaash to worship him. Take bath an hour before Sunset and then worship Shiv Jee. After the formal worship is completed, the story is told and then Mahaa Mrityunjaya Mantra is recited 108 times.
Skand Puraan relates a beautiful story about the greatness of this Vrat. Shaandilya Muni prescribed this Vrat to a Braahman widow who came to him with two boys - one was hr own son, Shuchivrat and the other on was an orphan prince Dharmgupt whose father was slain in a battle and his kingdom was overrun by enemies. All the three observed Pradosh fast and just after four months how they were rewarded.
Once there was an old woman. She had two sons one of her own Shuchivrat and the other one an orphan named Dharmgupt, whose father was slain in battle and the kingdom overrun by enemies. She was very poor. Once she met a sage named Shaandilya. Shaandilya Muni described her the importance of Pradosh Vrat. So she asked him the method to perform it. Acting upon the advice of the sage, the woman and the boys starting performing the Vrat with great devotion. After four months, that is, on the eighth Pradosh, Shuchivrat went to take bath in a river alone. There he got a pot of money. He took it and brought it home. He showed it to his mother. Mother was very happy to see it and asked him to divide between them, but Dharmgupt said - "I will not take other's person's fate. Everybody enjoys the fruit of his own Karm."
Thus one year passed observing Pradosh Vrat. One day the prince went to wander in the forest. As they were going here and there they saw many Gandharv girls playing around. Shuchivrat said - "We should not go forward from this point as these Gandharv girls are playing here. They can attract anybody. I do not like to go beyond this point." But the Prince Dharmgupt went forward fearlessly. When the Chief Gandharv girl saw the prince she got attracted to him. She sent away her friends to pluck the flowers and she herself waited for him to come to her. The prince when saw her alone, he also went to her and started talking to her. He told her that he was the Prince of Vidarbh Desh and his parents had died and the enemies had taken his kingdom. Then he asked her to give her introduction. She told him that she was the daughter of Vidravik Gandharv and her name was Anshumatee. She wanted to talk to him that is why she sent her friends away and remained alone here. She was very good at singing and she wanted that they should get married. She took out her pearl necklace and put it in the Prince's neck.
The prince asked her - "How will you be able to live with a Prince without his kingdom? And how will you come with me without the permission of your parents?" Anshumatee said - "Whatever will be the result, I will do what I like. Come day after tomorrow in the morning. I do not tell lies." and she went to her friends. Prince Dharmgupt also came back to Shichivrat and told him the whole story. Then the came back to their home and told her also the whole story. She was very happy to hear it.
The day after the prince went to the same forest again, and found Anshumatee sitting there with her father. Gandharv said to the Prince - "I went to Kailaash Puree. There Shiv Jee and Paarvatee Jee called me and told me that a prince named Dharmgupt has been wandering around without his kingdom, so go and help him to regain his kingdom. That is why I offer my daughter Anshumatee to you and I will help you to regain your kingdom to you. You will enjoy with this girl for 10,000 years and when you will come to Shiv Lok, she will still be with you in the same body. After saying this Gandharv married his daughter to Dharmgupt and gave him his Gandharv army too.
With the help of the celestial army the Prince won his enemies and entered his kingdom. he became the King and he invited his mother to his kingdom and gave a status of Queen Mother. Shuchivrat became his younger brother. Thus by observing Pradosh Vrat all got so much riches and kingdom. When Dharmgupt went to Shiv's supreme abode, Anshumatee also went with him with the same body. Thus so easily, and yet so greatly is Shiv pleased by this Vrat.