Srimad Bhagavatam and Vishnu Purana narrates the story of Samudra manthan where Devas and Asuras collaborated for churning the ocean to extract the divine nectar ( Amrut), which will give them immortality. Vasuki, the King of snakes, was used as the rope on the Mandara mountain which formed the churning rod. Due to strain caused by the churning, Vasuki emitted the dangerous poison called Halahala. The Frightened devas and demons prayed to Lord Shiva for help. Shiva who is the ocean of mercy, swallowed the poison to save the world. Goddess Parvathi, held his throat to stop the poison from inflicting his body, due to which his throat turned blue. He thus got the name Neelakantha. The Devas and Asuras praised and prayed to lord Shiva to grant them forgiveness from any mistakes that they have committed. Lord Shiva then came out of His unsteady state caused by the poison He had consumed, and danced his Anandathandavam between Nandi’s (Sacred bull) horns on 13th day of the Lunar month i.e. Trayodashi. It is this time which is known as Pradosha kaalam. During this pradosha kaalam, Lord Shiva is known to be in the most ecstatic mood and will grant all wishes, desires and wants of the devotees who pray to him.
Pradosha, in Sanskrit, literally means corrupt, bad or sinful. It also denotes the end of the day and setting in of the dark period ( i.e., night). It is in this period that Lord Shiva has been successful in saving mankind from the ill effects of the Halahala. Therefore, Pradosha Kaalam denotes, removal of evil for the devotees.
To those devotees who pray to him during the Pradosha Kaalam, Lord Shiva is known to be very generous giving away boons.
Observing Pradosham is an opportunity given to mankind to remove negative effects of karma. Shani Pradosham (Pradosham that occurs on Saturday) and Somavara Pradosham (Pradosham that occurs on Monday) are also called Maha Pradosham. On the day of Soma Pradosham devotees all across India, offer special prayers and pujas and observe fasting. Pradosha Vratam is good for spiritual upliftment and moksha (freedom from the continuous cycle of birth and death).
Somavara (Monday) is not only very auspicious for Lord Shiva but also for Mother Parvathi. Hence offering poojas on all Monday Pradosham helps a person to gain blessings of Lord Shiva and Devi Parvathi and negates the malefic effects caused by debilitated or afflicted Moon. The grace of Lord Shiva will remove diseases, enemies and destroy negative force of karma. Rudrabhisheka rituals done on a Saturday Pradosham or Monday Pradosham is said to be extremely auspicious. Abhishekam of the Lord is done with milk, curd, honey, ghee and coconut water. Flowers and bilva leaves should be used for the archana. Offering of coconut, fruits shall be done after dhoopa deepa. Body and mind comes under control by observing a fast, divine thought enters into the consciousness and makes the devotee pure( this is called Chittashudhi)
The Pradosha kaalam is between 5:00 pm to 7:00 pm. Lighting a lamp on the evening of Soma Pradosha Vratam is believed to be highly rewarding.
When you visit the temple of Lord Shiva, Pradakshinam (circumbulation of the Prakara) shall be done in a specified manner known as Soma Sutra Pradakshinam. Lord Shiva and Mata Parvathi will shower their blessings on the devotee performing such pradakshinam.