Om Namah Shivaye
Then nadudaya Sivane pottrri
En Natarkkum Eraiva pottri...
Pradosham pooja is performed on both Krishna paksham and Shukla pakksham Triyodasi .That is once in fifteen days,two days before Pournami and Ammavasai.
Timings 4:30 to 6:00 pm.
Praying in that time will free us from out sins and gives moksha finally (hence the name PRA dhosha). During pradosha time a special type of circumambulating called Soma sutra Pradakshnam.
Deavargal and asuraral jointly made an effort to extract amirtham from Kseearabthi (Par Kadal) using Serpent Vasugi (King of Snakes) and Manthara Hill.
During the process Vasugi exhalled Aaalahala ( Terrible poison ) even inhaling will be disastrous .
Devargal pleaded Lord Siva for rescue, then Lord Siva took the entire poison and gulped it.
Immediately goddess Parvathi held Lords neck so the poison will not go further. Hence Lords neck will be in blue colour , he is also called NeelaKandan.
Devargal was successful in extracting Amirtham on duadasi , they started celebrating forgetting the mercy of Lord Siva, the next day on Triyodasi they realised their mistake and went to Mount kailasa to plead for thier sin.
Nandi Devar stopped them with anger they circumbulated the mount in the shape of moon ( Chandran or Soma) hence we circumbulate like that during Pradosham.
Then Lord Siva was appeased and danced on the head of Nandi devar during prodosham hours (4:30 to 6:00 pm) this was on a Saturday so Sani Pradosham is very special.