Pradhosham is a bimonthly occasion on the thirteenth day of every fortnight in Hindu calendar. It is closely connected with the worship of Hindu god Shiva. The auspicious 3 hour period, 1.5 hours before and after the sunset is one of the optimum time for worship of Lord Shiva. Pradosha is indicative of day names in the calendar.
The fast or vow performed during the period is called "Pradosha vrata". A devotee should wear rudraksha, Vibhuti and worship Lord Shiva by Abisheka, Sandal paste, Vilva leaves, Fragrance, Deepa & Naivaedyaas (Food offerings)
Etymology of Pradosha - Pradosha is the son of Kalpa and Dosha. He had two brothers namely Nishita and Vyustha. The three names mean beginning, middle and end of night. The days from new moon day to full moon day is called "Sukla Paksha" and days from every full moon day to new moon day is called "Krishna paksha". During every month and during every paksha, the point of time when triyodashi (13th day of the fortnight) meets the end of dwadasi (12th day of fortnight) is called Pradosha.
During pradosha, Nandi (the sacred bull of Shiva) in all the Shiva temples in South India is worshipped. The festival idol of Shiva with Parvathi in a seated pose on Nandi is taken as a procession in the temple complex. The devas, celestial deities approached Shiva in the most propitious moments of pradosha to get relief from asuras - Danavas and Daityas. They ran around Kailasha, Siva's abode hitherto on a triyodashi evening and was aided by Nandi, Shiva's sacred bull. Shiva aided them in killing the asuras - the practise of worshipping Shiva on traiyodashi along with Nandi emerged and continues in Shiva temples. "Pradosha vrata" (vow) is performed on pradosham with sacred ritual steps following the tradition.
Shani Pradosha, the pradosha falling on a Saturday corresponding to planet saturn is considered important among other pradosham. The importance of Shani Pradosha is closely associated with Mahakaleshwar temple in Ujjain, a town in the central Indian state of Madhya Pradesh. The city of Ujjain was called Avantika and was famous for its devotional epicenter. It was also one of the primary cities where students went to study holy scriptures.
According to legend, there was a ruler of Ujjain called Chandrasen, who was a pious devotee of Shiva and worshipped him all the time. He was blessed with a celestial gem which could create miracles. Rivals of Ujjain, king Ripudaman and king Singhaditya of the neighboring kingdoms decided to attack Ujjain and take over the treasure. The king Chandransena unaware of the impending war was worshipping Shiva. He was joined by a farmer's boy named Shrikhar, who was walking on the grounds of the palace and heard the king chant the Shiva's name. However, the guards removed him by force and sent him to the outskirts of the city near the river Kshipra. Shrikhar continued to pray and the news spread to a priest named Vridhi. He was shocked to hear this and upon the urgent pleas of his sons, he started to pray Shiva inside the river Kshipra. The enemy kings chose to attack and it happened to be a Saturday and Triyodashi. With the help of the powerful demon Dushan, who was blessed by Brahma to be invisible, they plundered the city and attacked all the devotees of Shiva. Upon hearing the pleas of his helpless devotees, Shiva appeared in his Mahakal (form of light) and destroyed the enemies of king Chandrasen. Upon the request of his devotees Shrikhar and Vridhi, Shiva agreed to reside in the city and become the chief deity of the kingdom. From that day on, Shiva resided in his light form as Mahakal in a Lingam that was formed on its own from the powers of the Shiva and his consort, Parvati. It is believed that people worshipping Shiva on a Shani Pradosha would be free from the fear of death and diseases. They would also be granted worldly treasures.