Pradosha
Pradosha or Pradosham (IAST: Pradoṣa) is a bimonthly occasion on the thirteenth day of every fortnight in the Hindu calendar. It is closely connected with the worship of the Hindu god Shiva. The auspicious 3 hour period, 1.5 hours before and after the sunset is considered as the most suited and optimum time for worship of Shiva on this day. The fasting vow performed during the period is called "Pradosha vrata". A devotee should wear rudraksha, Vibhuti and worship Shiva by abhishek, sandal paste, Bael leaves, fragrance, deepa and naivedya (food offerings). Etymology. Pradosha is indicative of day names  in the calendar. 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. Legend. 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, Shiva'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. Observances. "Pradosha vrata" (vow) is performed on pradosha with sacred ritual steps following the tradition. During pradosha, the bull Nandi in all the Shiva temples in South India is worshipped. The festival idol of Shiva with his consort Parvati in a seated pose  on Nandi is taken as a procession in the temple complex. The Pradosha worship is done in the evening twilight or "sandhya kala." The performance of the "vrata" involves a fast followed by a vigil. A bath is taken one hour before sunset and the deities Shiva, Parvati, their sons Ganesha and Kartikeya, and Nandi are worshipped. Following this, Shiva is invoked. The Pradosha story is read out after the formal worship is concluded. Types. Shani Pradosha is the pradosha falling on a Saturday, while Soma Pradosha is the pradosha on a Monday. Maha Pradosha is the Pradosha which falls  before or on Maha Shivaratri in the Hindu month of Maagha. Shani Pradosha. 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.