If seeking blessings from Lord Shiva then the Pradosh Vrat is what you need to do as a devotee. In South India this vrat dedicated to Lord Shiva is generally known as Pradosham.
The lunar month is marked by both the Trayodashi Thithis. One is the Shukla Paksha Trayodashi and the other is Krishna Paksha Trayodashi. One may observe Pradosh Vrat on both these Tithis during the lunar month although some people are particular about the difference and stick to their choice of preference for the Pradosh Vrat.
Depending on the day the Pradosham falls on, its name changes. For instance, on a Monday it becomes Soma Pradosham, the Tuesday one is known as Bhauma Pradosham, while the Saturday Pradosham becomes Shani Pradosham. The starting of Pradosh Kaal begins once the sun sets in.
Therefore the best time for Pradosham Vrat is done when the Trayodashi Thithi occurs within the Pradosh Kaal. Puja for the diety Shiva is most auspicious when Trayodashi Thithi and Pradosh time cross each other.
Reference to Location for Pradosham Days:
Location plays a big role for decision regarding fasting day for Pradosham as the timing tends to differ from one city to the other even if both cities are within India. It all boils down to the sunset time happening in your area of location. Please note that Trayodashi Thithi must prevail after sunset for someone to take part in Pradosham fasting. Therefore it is allowed to undertake Pradosham fasting one day prior to Trayodashi Thithi which is known as Dwadashi Thithi. For a detailed insight into Prashodam days based on various locations of India, kindly look up our Hindu Calendar.