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.