Saturday is a great day to worship Kali or Bhairo or to invoke dark spirits. Interestingly in Shaivism the most inauspicious tithi charted out by Vishnavas is considered the most auspicious tithi. In Shaiva texts Krishnachaturdashi is said to be the most auspicious tithi for worship of Kali or such deities. In the Tara shaktipeeth of west Bengal Saturday is regarded the best day to do have darshan and worship in the temple. The great Kali devotee Ramakrishna Paramhansa would instruct disciples to organize special puja of Kali on Saturday or on Chaturdashi. Saturday is good for worshipping of dark natured deities, good for Tantric jadu-tona. Satanist world over perform their evil work on Saturday. Saturn is good for inflicting pain, do evil on others or on one oneself. All kinds of evil work can be done on Saturday.
But in astrology Saturday is the most inauspicious day because it is ruled by Saturn called Mand, who is a slow moving planet. The description of Saturn itself is very inauspicious –

पिंगेक्षण: कृष्णवपु: शिरालो मूर्खोऽलस: स्थूलनखोनिऽलात्मा।
क्रोधी जरावान् मलिन: कृशांग: स्नाय्यवातत: सूर्यसुतोऽतिदीर्घ:।
Saturn has pingal (yellowish) eyes, he is black complexioned with large nails, thik veins, thin body and muscles are wide. He is old aged, obtuse, lazy and angry.

Another text says that Saturn has hard nails, teeth and hair. He is mayavi, has large hands and leg. He is airy bilious. He has prominent veins, lazy and having no peace.
Puranic stories made him even more evil. In stories, he casts his glance and things are destroyed; even son of God Shiva ‘Ganesha’ is not spared.

Someday a friend visited to a South Indian Astrologer. He had problem fixing marriage of his daughter. Astrologer asked him about birth chart of his daughter, he handed it to him. Astrologer said ‘leave it to me, I will work out and let you know. I would not like to delay’ When his friends went away. He thought, ” I myself is running Saturn period and chart is handed over to me today and it is Saturday’. This was already a sign that marriage will be delayed.

Saturn is delaying planet and if it is involved in any matter, matter will not be denied but it will be delayed for sure. Suffering (दुःख ) of all kind is the work of Saturn. In its major period it is experienced by even kings. Natives born on Saturday often suffer in their life comparatively more than the natives born on auspicious days. As for example this chart of Henry Miller. He was born on Saturday and unfortunately he ran Saturn period from 1907-1926. He mostly suffered in this period. But even after his first publication his life was not very stable and very peaceful, though he had several affairs and pleasure from women.

If birth is on Saturday and tithi is also inauspicious, say Krishnachaturdashi or Amavasya or any such tithi, and Moon is in bad nakshatra then it will be called inauspicious birth and native will suffer in life. If in Ascendant Saturn’s sign is rising, birth is on Saturday and tithi is also inauspicious then native will suffer. Generally their life is not normal and they get stability in life very late. In such cases If birth chart is heavily afflicted, native commits’ crime or commits suicide. In above cases if native is women, she will not have peaceful married life or she may tread evil path. In many cases such natives are raped or murdered or commit suicide, I have seen this in hundreds of charts. In the birth chart of majority of Nazi leaders you can also find it. Most of the Nazi leaders were born on inauspicious tithi and their chart is heavily afflicted. Hitler himself was born on Saturday in Krishna-Shashti tithi, his chart is heavily afflicted. He was one of the greatest criminal, a demon and he committed suicide.