What is Sade Sati?
Sade Sati is the roughly seven-and-a-half year period when Saturn (Shani) transits three signs around your natal Moon — the 12th house from your Moon sign, the Moon sign itself, and the 2nd house from it. Saturn spends about two and a half years in each sign, so the full journey spans around seven and a half years, which is exactly what "Sade Sati" means.
Crucially, Sade Sati is judged from your Moon sign (Rashi), not your Sun sign. That is why two people born under the same Western sun sign can be in completely different phases of Sade Sati.