Pradosh Vrat (Pradosham)
Pradosh Vrat is a fast to Shiva kept on the 13th tithi (Trayodashi) of each paksha, twice a month. It centres on the evening twilight, pradosh kaal, so the day is set by the tithi at sunset.
Also known as: Pradosha vratam
Pradosh means the evening twilight around sunset. Pradosh Vrat falls on Trayodashi, the 13th tithi, in both the bright and dark halves, giving about 24 a year. Devotees fast during the day and worship Shiva, often with Nandi, in the pradosh kaal window around sunset.
Because the worship is in the evening, the vrat is kept on the day when Trayodashi prevails at sunset. That can be a day the panchang labels Dwadashi by its sunrise tithi, which is why a Pradosh date sometimes looks a day early. A Pradosh on Monday is Soma Pradosh, on Tuesday Bhauma Pradosh, and on Saturday Shani Pradosh.
It is a devotional observance rather than a muhurat for other activities, and families adapt the fast to health and age.