The law of chametz turns on timing. Sifrei Devarim asks exactly when eating leaven must stop before Pesach.
Our source for this particular detail is Sifrei Devarim, a collection of legal interpretations on the Book of Deuteronomy. In section 130, it quotes Deuteronomy (16:3): "You shall not eat chametz upon it." But what does "upon it" mean?
Rabbi Yehudah, a prominent Tanna (a sage whose teachings are recorded in the Mishnah (the earliest code of rabbinic law)), offers an explanation. He says that the prohibition against eating chametz on erev Pesach, specifically, begins at the sixth hour of the day. The text derives this from the phrase "upon it", interpreting it to mean from the time of the sacrifice of the Paschal offering. So, when did that happen? From the sixth hour onwards. The Paschal lamb, the Korban (a sacrificial offering) Pesach, was sacrificed in the afternoon. This sacrifice was a central act of preparation for the Seder meal that would be eaten that night. Rabbi Yehudah connects the prohibition of eating chametz to that specific timeframe.
It's a reminder that even in the midst of all the hustle and bustle of Passover preparations, timing matters. Each moment leading up to the Seder is imbued with meaning, and even the seemingly small act of refraining from chametz at a specific time connects us to the ancient rituals and profound symbolism of the holiday.