According to the Skanda Purana, on the day of Triyodashi on both sides of each month, the time of evening is called "Pradosha" and on this day Pradosha fast is kept in order to please Shiva. Pradosh Vrat Puja is observed on both Trayodashi Tithis, i.e. Shukla Paksha Trayodashi and Krishna Paksha Trayodashi, in lunar month. Some people distinguish between Shukla and Krishna Paksha Pradosham.
The story of Pradosh Fast is the following:
According to a story described in Skanda Purana, in ancient times, a widowed Brahmin would take alms against his son and return to the evening. One day when he was returning from alms, he saw a beautiful boy on the river side, who was Prince Dharmagupta of Vidarbha. The enemies had killed his father and grabbed his kingdom. His mother's death was also a famine. The Brahmin adopted and brought up the child.
After some time the Brahmani went to Dev Temple with the two children. There he met Rishi Shandilya. Rishi Shandilya told the Brahmin that the boy whom he has received is the son of Raja of Vidharbha who had been killed in battle and his mother had made his meal. Rishi Shandilya advised Brahmini to fast and perform Pradosha fast. By the command of the sage, both the children started performing the Pradosha fast.
One day, while the two boys were roaming in the forest, they saw some Gandharva girls. Brahmin child returned home but Rajkumar Dharmagupta started talking to Gandharva girl named "Anshumati". Gandharva Kanya and Rajkumar became fascinated by each other, the daughter called the Prince to marry her father to get married. On the second day when he came again to meet Gandharv, the father of Gandharva Kanya told that he is the prince of Vidarbha country. Under the command of Lord Shiva, Gandharvraj married his daughter to Rajkumar Dharmagupta.
After this, Prince Dharmagupta reciprocated the Vidarbha nation with the help of the Gandharva army. All this was the result of the Pradosha fasting of Brahmani and Rajkumar Dharmagupta. According to Skand Puran, the devotee who listens or reads the Pradosha fast after Eklavya after Shiva Puja on the day of Pradosh vrat, has no meanness till hundred births.
Pradosh Vrat Benefits
The advantages of Pradosh fasting is described in Shiva Purana. It is said that the individuals who kept pradosh fasting should be honored with thriving, youngsters, peace and satisfaction in the families. Ladies who look to have descendants watch this vrat so intensely and are honored with youngsters. It is likewise trusted that those kept this vrat constantly will be liberated from their transgressions.