Pradosh Vrat or Pradosham is observed to seek blessings of Lord Shiva. It 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 a 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 lists this time for all Pradosham and this time should be preferred for doing Shiva Puja.
Location-based Pradosham Days
The day of fasting for Pradosham can differ for two cities even if the cities are within the same state. Fasting for Pradosham depends on the 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.