Pradosham is the time when Lord Shiva consumed the Hahahala poison that emerged out during the churning of the milky ocean. The poison could not be handled by anybody. So out of deep compassion, Lord Shiva consumed it and danced in bliss in the form of Nataraja on the head of His divine bull vehicle Nandi between 4.30pm – 6 pm on the thirteenth phase of moon. 
So every thirteenth phase of moon evening Lord Shiva radiates the energy to destroy the negativity thereby saving us and protecting us.

Also it on the Pradosham time Shiva delivered the secret meditation techniques for Enlightenment in the book Vijnana Bhairava Tantra to Devi. 

On this day abishekam is offer to the divine bull vehicle of Shiva, Nandhi.