Pradosh Vrat, which is also known as Pradosham in South India, is observed to seek blessings of Lord Shiva.
Pradosh Vrat 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.
When Pradosham day falls on Monday it is called Soma Pradosham, on Tuesday it is called Bhauma Pradosham and on Saturday it is called as Shani Pradosham.
Pradosha vrata is a Hindu vrata for the worship of Lord Shiva and Parvati. The Pradosha worship is done in the evening twilight or sandhya kala on the Trayodashi of both lunar fortnights (Shukla and Krishna Paksha). These are the 13th tithi, or lunar days, from the New Moon (Amavasya) and Full Moon (Poornima).