What is Shani Pradosham and Maha pradosham?

Aum Namah Shivaya......

Mahaa Pradosh Kalam comes yearly during the month of Maagh/Maashi (Uma +ShiVa) before Mahaa Shiv Raatri. If the Mahaa Pradosha comes on Saturday, it is called Shani Mahaa Pradosha. Saturday is the day when Lord Shiva swallowed poison and this is called Shani (Sthira in Samskritam) Pradosha. It is categorized into three categories of importance depending upon the month on which Pradosha falls:

Uttama Sthira Pradosha - Maximum power - The Sthira Pradosha that comes during the months of Medam, Yedavam, akh,Karkadikom, Vruschikam, during the Growing Lunar Cycle (Shukla Paksha), i.e from New to Full moon period.
Madhyama Sthira Pradosha - Normal power - The Sthira Pradosha that comes during the months of Medam, Yedavam, akh,Karkadikom, Vruschikam, during Diminishing Lunar Cycle (Krishn Paksha), ie Full to New moon period.
Adhama Sthira Pradosha - Minimum power - All the other Sthira Pradosha. The Pradosh that falls on any Saturday is called MahaA Pradosha (Sthira Pradosh).
It is believed that each day of the week has different benefits during Pradosh time:

Sunday (Bhaanu Vaara Pradosha)
Peace to all (Mangalam to all)
Monday (Indu Vaara Pradosha)
Positive (perception) thinking
Tuesday (Bhauma Vaara Pradosha)
No Poverty (Prosperity to all)
Wednesday (Saumya Vaara Pradosha)
Blessings with progeny, Knowledge and education
Thursday (Guru Vaara Pradosha)
Divine blessings through the Pitru's, all dangers will be eliminated
Friday (Bhrgu Vaara Pradosha)
Nullifying (negativeness) opposition
Saturday (Sthira Vaara Pradosha)
Elevation (promotion) getting back the lost wealth.
Shani Pradosham and Soma Pradoshams occurs when one of the 13th moon days occurs on a Saturday or a Monday respectively. The planet Saturn is considered an Agent of Karma who delivers many of life's tougher lessons to help us refine and evolve our souls. Pradosham time are measured by the phases of the Moon.