Pradosh Vrat 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.For Pradosham Vrat, day is fixed when Trayodashi Tithi falls during Pradosh Kaal which starts after Sunset. The time window after Sunset when Trayodashi Tithi and Pradosh time overlaps is auspicious for Shiva Puja. DrikPanchang.com lists this time for all Pradosham and this time should be preferred for doing Shiva Puja.
Location based Pradosham Days
It is important to understand that fasting day for Pradosham might differ for two cities even if those cities are within same Indian state. Fasting for Pradosham depends on sunset time and it is observed when Trayodashi Tithi prevails after sunset. Hence Pradosham fasting might be observed on Dwadashi Tithi i.e. one day before Trayodashi Tithi. As Sunset time is different for all cities, it is important to refer Hindu calendar like this website which lists location based Pradosham days. As generating location based dates are time consuming most sources ignore this fact and publish single list for all cities.