Although the prohibition against ĥametz applies primarily during the seven days of Ĥag Ha-matzot, from the fifteenth through the twenty-first of Nisan, nevertheless we were commanded to remove ĥametz from our homes at noon on the fourteenth of Nisan, Erev Pesaĥ.
The prohibition against eating ĥametz also begins at noon on the fourteenth, as is written, “And you shall sacrifice the Pesaĥ [offering] to G-d … you shall not eat ĥametz with it” (Devarim 16:2-3). This prohibits the eating of ĥametz from the time fit for bringing the Pesaĥ sacrifice, i.e., at noon on the fourteenth of Nisan. This prohibition against eating ĥametz includes the prohibition against deriving any benefit from it.
In order to distance one further from possibly violating commandments, the Sages added to the prohibitions and forbade gaining benefit from ĥametz for an additional hour. They also forbade eating ĥametz for two extra hours, since on a cloudy day people are likely to err by as much as two hours.
These times are calculated by dividing the day into twelve equal parts, each of which is called “a seasonal hour” (“sha’ah zmanit”). Thus, one may eat ĥametz for the first four seasonal hours of the fourteenth. During the fifth hour it is rabbinically forbidden to eat ĥametz, but it is permissible to derive benefit from it by, for example, feeding it to an animal or selling it to a gentile. When the sixth hour of the day begins, possession of ĥametz becomes prohibited rabbinically, and if one forgot to sell it to a gentile, it must be destroyed. When midday arrives, that is, after the sixth hour ends, ĥametz is forbidden by Torah law both for consumption and for deriving any benefit, and one must dispose of it as soon as possible. Every moment that one does not get rid of it, he violates the positive commandment to remove the ĥametz (see below 3:6 concerning the mitzva of removing the ĥametz).
Once the holiday begins, two additional prohibitions apply: bal yera’eh and bal yimatzei. The prohibition against eating ĥametz also becomes more severe: one who willfully eats ĥametz after midday on the fourteenth is punishable by lashes only, whereas one who willfully eats ĥametz after the holiday begins is punishable by extirpation. This is based on the verse: “whoever eats ĥametz, from the first day until the seventh day, that soul shall be cut off from Israel” (Shemot 12:15).
Ĥametz becomes permissible once again after Pesaĥ, except that our Sages forbade ĥametz that belonged to a Jew during the holiday (“ĥametz she-avar alav ha-Pesaĥ”). Since by keeping the ĥametz on Pesaĥ he violated the prohibitions of bal yera’eh and bal yimatzei, the Sages prohibited eating or gaining benefit from that ĥametz even after Pesaĥ. Ĥametz that was in the possession of a gentile during Pesaĥ, however, is permissible; a Jew may buy it and eat it (SA 448:1-3).
The prohibition against eating ĥametz and obtaining any benefit from it applies from noon on the fourteenth, according to R. Yehuda, as explained in Pesaĥim 28a. Rambam, as well as R. Yitzĥak ibn Gi’at, Rosh, and most Rishonim, ruled in accordance with this view. R. Shimon’s view is that the prohibition against eating ĥametz begins with the holiday itself, and it is only the mitzva of removing the ĥametz that comes into effect at noon on the fourteenth. Some Rishonim rule in accordance with this view, although they disagreed about what the mitzva of getting rid of ĥametz entails according to R. Shimon. Let us mention two approaches. According to Ramban and Raavad, since one must get rid of the ĥametz, he is also not allowed to eat it; yet Torah law permits him to benefit from it in the course of its being burned. Only the Sages prohibited deriving any benefit from the ĥametz from the beginning of the sixth hour of the day. According to Ha-ma’or, the mitzva of disposing of the ĥametz does not imply a prohibition against eating it, since by eating it, one is, in fact, disposing of it (see Encyclopedia Talmudit, vol. 16, s.v. “ĥametz”, p. 66; Hilkhot Ĥag Be-ĥag, p. 8, n. 3). Again, the view of most poskim is that the halakha follows R. Yehuda’s view that the prohibition against eating ĥametz and the prohibition against deriving any benefit from ĥametz are of Torah origin, and apply from noon. This is how the halakha is decided in SA (443:1).