If something happens wrong with you we say sate sati starts of the person.
As per the wiki, Its the period of Sade-sati starts when Saturn enters the zodiac sign immediately before the zodiac sign of Moon at the time of the birth of the individual. That is, if the Moon sign (Ayamsha) at the time of birth of the native was Taurus, then the Sadesati will begin when Saturn enters sign Aries. The Sadesati will continue while Saturn transits over this sign and the next two signs, i.e. the birth sign and the sign after it. Saturn spends around 2 1⁄2 years in each sign. To cross these three signs it takes about 7 1⁄2 years. Thus the name Sadesati which literally means seven and a half. This is given by wikipedia.
But many people uses for bad time. if someone not doing good or wrong happens with person we say sadesati starts of the person. we use it for bad happenings of the person.