Pradosh Vrat is observed on the 13th day of a fortnight and there are two Pradosham in a Hindu month. Lord Shiva and Goddess Parvati are worshipped in the evening during twilight on this day. (on the Trayodashi of both lunar fortnights (Shukla and Krishna Paksha).)
Monday fasts or Solah Somvar vrats are kept for 16 consecutive Mondays. These fasts are dedicated to Lord Shiva.
Solah Somvar Vrat Katha : सोलह सोमवार व्रत कथा
Somvar or Monday Vrat is one of the prominently known fasting practices observed in the Hinduism. Somar vrat is dedicated to worshipping and propitiating Lord Shiva and Mother Parvati. All Mondays are devoted to the worship of Shiva as this day is sacred to Lord Shiva.