Pradosh fast is observed by people on the 13th day of the lunar calendar which is also known as the Trayodashi tithi…most commonly known as the maas Shivratri, of the Krishna Paksha and Shukla Paksha, as per which, Pradosh comes twice in a month. As the name suggests, Pradosh has two meanings…it literally means removal of sins and secondly, its the Dusk time more commonly known as the Godhuli…a time period of about 3hours, divided into 1.5hrs before the sunset and 1.5hrs after sunset that’s known as the Pradosh Kaal.
As we all know that during Samudra Manthan, the venom that came out along with the the Amrit was taken by lord Bholenath and Ma Parvati kept Her hand on His neck to prevent that lethal poison to enter His body, that turned the color of His throat blue after which Bholenath was also called Neelkantha. Post that event, Devas and Asuras got the Amrit along with new life to the universe. They all in the happiness and festivity forgot to thank Lord Shiva. But on the thirteenth day (Trayodashi), they realized their mistake and asked for mercy. Lord Shiva forgave them and celebrations continued. It is believed that that time was Pradosham.