Pesachim 5 - Why stop eating chametz already before Passover?
We mentioned earlier that we start destroying chametz the day before Passover , because it is prohibited already then. Why? Here is Abaye’s explanation.
The Torah said, " For seven days chametz should not be found in your homes ." However, it also said, " But on the first day you should destroy chametz ." What is there to destroy if on the first day you can’t even find it? It must be, therefore, that by "first day" in this context the Torah means "prior" day. Very well, but if so, it should be the whole day!? - that is the meaning of the word "but," which comes to limit the new rule, and it is logical to limit it to a half-day. Thus, the clear result: chametz is prohibited to be eaten after midday before Passover, but the more serious consequence of being cut off from one’s people for one who eats chametz - that starts only when the Passover really begins.
The Talmud then finds many more ways to prove that chametz is prohibited half a day prior to Passover. This need is probably because neither all of the proofs are based on a complicated derivation and none of them is obvious.
Art: A Weaver's Workshop With A Couple Eating At A Table by Cornelius Decker (1618-1678)