Greetings of the day!!
There are 12 zodiac signs named:-
For example if your moon sign is no-3, gemini and saturn is transitting in taurus sign then your sadesati will start the first phase then when it will transit in Gemini that will be second phase and the last phase will be transitting in Cancer zodiac Sign
Transit of Saturn in each zodiac sign is 30 months that is 2.6 (2.5)years, so transit in 3 zodiac sign = 2.6(2.5) x 3=7.5 years
Sade-Sati is the 7 1⁄2 years long period of Saturn (Shani). This astrological phase is much feared by those in India. According to those beliefs, this is a period with many challenges.
If Saturn is ill-placed in bad houses, he may face challenges that reflect this bad placement.
Another related astrological transit of Saturn is the Dhaiyya (2.5 years), also known as Small Panoti (Small Trouble), and Kantakshani, or Ashtamshani. Vedic astrology says that these two are hazardous periods, though not as much as the Sade-sati.