And why did the taking of the Pesach precede its slaughtering by four days? Because Israel was stepped in idolatry in Egypt, which countervails all of the mitzvoth, as it is written (Numbers 15:24) "And if from the eyes of the congregation it (idolatry) were done unwittingly, etc." Scripture singled out this (idolatry, as tantamount to transgression of all of the mitzvoth [viz. Ibid. 22]).