Chabad.org — English translation + Rashi
Torah Temimah (Pesachim 6a): from the early instruction concerning Pesach, Chazal derive that one asks and expounds the laws of Pesach thirty days before Pesach.
English: As it was taught: one asks and expounds the laws of Pesach thirty days before Pesach. Rabban Shimon ben Gamliel says: two weeks.
Gezerah / temporal preparation — the early announcement itself becomes the model for the thirty-day study window.
Preparation for the festival begins well before the day; the Torah’s own narrative supplies the precedent.