Pradosh Vrat is also known as Pradosham, and is dedicated to Lord Shiva. The Pradosh Vrat falls on the thirteenth day of every fortnight in the Hindu calendar. Pradosham is known to be the auspicious three hour period of twilight which is considered as the best time to worship Lord Shiva. The devotees of Lord Shiva can also fast on this day. This fact itself is known as Pradosh Vrat. On Pradosham, people also wear Rudraksha and Vibhuti in order to perform the traditional rituals and worship Lord Shiva. Many people also use sandal paste, bael leaves for this pooja.
As per the Puranik legend, many Gods appealed to Lord Shiva in one of his most conducive Muhurta of Pradosh to help them in defeating the Asuras. The Gods approached Shiva on Mount Kailash on a Pradosh evening to ask for his help. These Gods were received by Nandi who is Lord Shiva's sacred bull. Shiva agreed to help the Gods in defeating the Asuras. This is why the tradition of worshipping Lord Shiva along with his sacred bull Nandi started on Pradosh and is still followed today.
According to another legend, Pradosh is considered as the time when Lord Shiva regained consciousness after he consumed the poison from Samudra Manthan and did 'Tandava'. It is also believed that at the time of Pradosh, Lord Shiva is in a happy state and he blesses everyone who prays to him.
According to legend, in ancient times, there used to be a wealthy man in the city. He had all the riches, but had no children. The man and his wife always wanted children and so they decide to go on a pilgrimage handing over all his work to servants. On their way, they meet a monk who was meditating. Seth thinks of taking the monks blessing before moving further. The monk's meditation is interrupted. He smiled at Seth and his wife who were sitting in front of him. The monk suggested to them that they should observe the Shani Pradosh fast and their wish will be fulfilled. On returning from the pilgrimage, the wealthy man and his wife observed the Shani Pradosh fast which fulfils their wish and after some time they are blessed with a child.