According to the Shiva Puranas, observing a fast on Pradosham Vratam is regarded as highly auspicious and beneficial. One would be blessed with wealth, children, happiness and honor. While the fast is undertaken by all Shiva devotees, but all women who have been longing for a son specially observe this. Also, praying to Lord Shiva during the course of the fast is said to liberate the devotee from all his previous and present sins and evil deeds committed.

According to one legend, Lord Shiva is believed to have drunk the Halahala poison during Pradosham. This poison was mixed with the Ocean of Milk. (Samudra Manthan) Another myth relates the importance of observing this vrat with the divine couple, Lord Shiva and Parvati. On the Trayodashi day during the evening twilight, the couple is said to be in a propitious and favorable mood. Hence, all prayers and grants demanded by a sincere and loyal devotee are easily fulfilled. Additionally, Bael or Bilva leaves are offered to the Lord during this twilight period as it is regarded to be very fortunate. Hence, all stern Shiva devotees observe both the fasts that occur on two Pradosha days in a month. Such devotees consume only water during their fast and break their fast by eating Prasad after the evening prayers only.