Shani Pradosh, or Sani Pradosham, is the Pradosha Vrata falling on Shanivar (Saturday). Pradosh is dedicated to Lord Shiva. Shaniwar, or Saturday, is dedicated to Shani, one of the Navgrahas. Shani is widely believed to be a troublemaker and it is believed that fasting and praying on Pradosh day will help in keeping the harmful effects of Shani away.
Pradosh, or Pradosham, falls on the 13th day (during the waxing and waning phase of moon) in a traditional Hindu lunar calendar. It is believed that Lord Shiva and Parvati are in playful mood during this period and therefore they will readily fulfill the wishes of the devotees.
It is believed that people under the bad influence of Shani can escape it by praying to Shiva on the day.
You may also like to read