Pradosham occurs in Thrayodashi thithi in both Shukla Paksha and Krishna Paksha and thus occurs twice in a month. Those who pray to Shiva during this time get relieved of their human bondage and attain mukthi.
Krishna Paksha Pradosham in the month of Margazhi falls on January 14, 2018.
Pradosha Kalam is between 4.30 and 6 in the evening and we know that all Shiva temples conduct special poojas during this time. Lord Shiva fulfils the wishes of those who pray to Him during this time. People offer milk, curd and honey for anointing Shiva during Pradosham and also offer flowers and garlands for decorating the Lord.
There is a mythological reference to Pradosha Pooja. While trying to get amrutham from the ocean of milk using the serpent Vasuki and Manthar hill, the Devas came upon Halahala poison which was eaten by Lord Shiva in order to rescue them.
As per the order of Shiva, the Devas continued their attempts to get amrutham and got it on Dwadashi (12 TH Moon day). They forgot to thank the Lord as they were overwhelmed with happiness. Next day on Thrayodashi, on the 13th Moon day, they realized their fault and pleaded with Lord Shiva to forgive them.
The graceful Lord forgave them and danced in between the horns of Nandi (the divine bull), being pleased with their prayers. This time is called Pradosham.
Observing Pradosha vrata is very auspicious. It helps to remove karma or karmic energies that limit our potential. During Sani Pradosham in any of the months of Aippasi, Karthigai, Chithirai, and Vaikasi, you can start Pradosha Vrata. (When Pradosham falls on Saturday, it is called Sani Pradosham).
One should not eat in the daytime. During Pradosha Kalam in the evening after taking bath and partaking in the special poojas at the temple, one can break the fast and eat with the devotees of Shiva. Eating, sleeping, bathing, applying oil, doing mantra japa, embarking on a travel and reading should not be done during Pradosha Kalam.
Pradosham is an auspicious period as all the hosts of celestial beings and gods come down from the heavens and attend the worship in their subtle forms. Lord Shiva is known to be in the most ecstatic of moods.