When 13th moon falls on Saturday. The planet Saturn is considered an agent of karma who delivers many of life’s tougher lessons to help us refine and evolve our souls. Pradosham time are measured by the phase of the moon.
Saturday is dedicated to lord Shaniswaran, one of the navagrahas, or nine celestial gods. During pradosham time, abhishekam done with turmeric, kumkum, honey, rice powder and etc is considered to give fruitful benefits.
To overcome the bad influence of shani, devotees observe a vratham on shani pradosham and offering prayers to lord shiva during the day.
It is a very auspicious day to perform Navagraha homa or Shivapooja on this particular day.