Pradosham (also called Pradosh Vrat) falls on the thirteenth Tithi (Trayodashi) of each lunar fortnight -- both Shukla and Krishna Paksha. The word "Pradosha" refers to the twilight period, and the vrat is observed in the evening during the twilight hours. Pradosham is dedicated to Lord Shiva and is considered highly auspicious for Shiva worship. When Pradosham falls on Saturday, it is called Shani Pradosham, and when on Monday, it is called Soma Pradosham -- both are considered especially powerful.