Sade Sati is the period when Saturn transits the three signs around your natal Moon: the 12th from it, the Moon's own sign, and the 2nd from it. Saturn takes about two and a half years to cross a sign, so three signs is roughly seven and a half years. Because Saturn's orbit is about 29 to 30 years, the passage returns roughly every three decades, and most people meet it two or three times in a lifetime.
It is counted from the Moon, not the Sun
This is the single most common error. A newspaper column that assigns you a rashi from your date of birth alone is usually using the Sun. Sade Sati is calculated from the sign the Moon occupied at your birth, which needs the time of birth to establish. If a site tells you Sade Sati is running and another says it is not, check which one asked for your birth time.
First phase — Saturn in the 12th from the Moon
The 12th house is expenditure, foreign matters, hospitals and sleep. This phase tends to show as costs that arrive without warning, disturbed sleep, and a general sense that the ground is less certain than it was. The pressure is mental before it becomes material. People often report this phase as anxious rather than difficult.
Second phase — Saturn on the Moon
This is the heaviest stretch, and the one the reputation is built on. Saturn sitting on the natal Moon presses directly on the mind and the body at once. Health, family duty and career all tend to demand endurance in the same period. What makes it hard is rarely a single event; it is the absence of a break between them.
Third phase — Saturn in the 2nd from the Moon
The 2nd is family wealth, speech and what is kept rather than earned. Money tightens here and family obligation weighs, but the character of the period is different: the load is visibly lifting. Most people describe the third phase as tiring rather than frightening.
What Saturn is actually doing
Saturn removes what is not carrying its own weight — a job that stopped paying its way, an arrangement held together by habit, sometimes a relationship. What survives the passage is generally sounder than what went into it. The people who find Sade Sati worst are usually those who spend it trying to hold everything in place unchanged.
Shani Dhaiya is a different, shorter transit — two and a half years with Saturn in the 4th or 8th from the Moon. It is milder and often confused with Sade Sati.