As Shani (Saturn) is malefic (Paapa Graha) by its nature, thus Saturday generally is considered to be a day for Paapa Carmas (sinful, unethical, cruel actions) only if certain benefic work isn't specified for it. Its nature is known as "sharp" or "horrible" (in ancient Sanskrit - "Teekshna" or "Daruna"), thus all such works are good on Saturday. In combination with "sharp" Nakshatras (Jyeshtha, Ardra, Ashlesha and Moola) activity related to these Nakshatras will be very fruitful. As for effectiveness - Saturday is effective during night-time. This day is auspicious for everything related to blue or black color, working with Carma, systematization works, bringing something into order or system, works related to old subjects or people, agriculture works (as Shani, a ruler of this day, rules Carma, time, experience, systematic approach, old people, common people (lower castes), agriculture, Vata-Dosha, thieves, liars, servants etc.). All works that somehow are related to Yoga, Pranayama, "control of death" are very auspicious at this day (as Yama, deity who's responsible for human's longevity, is a tutelary Devata of Saturday. It's very auspicious to appeal to Yama while trying to do something important during Saturday). Adhi-Devata of Saturday is Brahma, deity responsible for creation and knowledge. Thus, Saturday is a very special day for any actions related to getting knowledge as experience of sires. Also it's a good day for getting initiatory Mantras of Yagya (a ceremony in which oblations are offered to certain Devatas).
Basing on mentioned parameters works in following fields are especially prescribed for Saturday: entering a new house & city, trading with deceit & forgery, telling lies, dreadful deeds, insurgency, works related to obstruction, trading of arms & ammunition, poisoning, bereft of courtesy, usage of iron & steel, arm & armory, dead, servants, debased persons, old persons, thief, buffalo, camels, birds, trapping of elephant, stable works, profession related to capturing, collection of houses, departure, broken utensils, glass, stones, learning secret Mantras from Guru (Guru Deeksha), service.