Transit of Saturn in the 12th House, Ascendant and the 2nd House with reference to the Moon Chart is termed as Sade Sati. Saturn takes two and a half years to travel in one sign therefore to travel three Houses 12th, Ascendant and 2nd House it takes seven and a half years. Therefore this Transit of Saturn in the three Houses 12th, Ascendant and 2nd with reference to the Moon chart is called Sade Sati. Sade sati is considered extremely inauspicious. This is because Saturn is considered as one of the most dreaded Planet in India by the believers of Astrology and its Transit is considered inauspicious* because it will disturb Planet, Moon which is Natural Significator of Mind.
In Hindu Astrology Transits are considered to pin point the events after Dasa, Bhukti, Antar have endorsed an event. Therefore if the Dasa, Bhukti Antar are good and Saturn is even bad can not do any harm in whichever House it is transiting in the Horoscope. Moreover Transit of single Planet should not be considered. Transits are handled in following manner :
Any of the three planets which signify a particular event should
either conjoin in one house
or aspect each other
or transit the significator houses
or aspect them
or transit over their natal positionn
This implies that at least three Planets are required to trigger an event after the Dasa, Bhukti, Antar have endorsed it. Therefore the discussion on Sade Sati becomes worthless.
* Auspiciousness and Inauspiciousness are terms related to religion and are comparitive words therefore reaching to conclusion is not possible.
Pandit Jawahar Lal Nehru became the Prime Minister during Sade Sati. You will find several examples of people who have excelled in various fields during what is being advertised as Sade Sati.
CONCLUSION
According to Hindu Astrology an event Takes place as follows:
1) Dasa, Bhukti, Antar have endorsed it.
2) After that Transits of Planets are considered to further pin point the same. At least three Planets are required which signify the same event to finally trigger the event.
A single Planet is not capable to culminate an event therefore Saturn single handedly in its Sade sati will prove ineffective if it does not get support from at least two more Planets each in Dasas and Transits.
Note : You may find bad results in a Horoscope during Sade Sati of a Native. Do check the effects according to the rules given in Hindu Astrology. Do not jump to the conclusion that because Sade Sati is in operation bad results are operating or have operated.