Pradosh Vrat is observed on Trayodashi, the thirteenth tithi of both fortnights, in honour of Shiva. Pradosha kala is the twilight period after sunset, when Shiva is believed to dance in joy; worship is offered at this time.
When Trayodashi falls on Monday it is Soma Pradosh, on Tuesday Bhauma Pradosh, and on Saturday Shani Pradosh. The vrat is kept on the day the Trayodashi tithi prevails during the evening pradosha period.