Pradosham is an auspicious 3-hour period, 1.5 hours before and after the sunset, and is observed bi-monthly on the 13th day of every fortnight. The day is considered ideal for the worship of Lord Shiva and Nandi (Bull). When a Pradosham falls on a Saturday, it is called as Sani Pradosham. Lord Saturn, the planet of Karma rules Saturday, and worshipping him and offering your prayers to Lord Shiva on a Sani Pradosham is believed to resolve your sins and karmic effects.
To appease the Lord and avoid such ill effects, people usually keep fasting for Lord Shani on Saturdays. When a Pradosham falls on a Saturday, worshipping both Lord Shiva and Shani on that day adds more divinity.
Book a professional vedic priest for performing rudrabhishekam on pradosham days.