The Sages forbade eating matza on Erev Pesaḥ, so that it is beloved to us when we eat it at night and to distinguish between matza eaten before Pesaḥ and the matza eaten as a mitzva during the Seder. This prohibition applies even to children who understand the meaning of the matza, which commemorates the Exodus from Egypt. It is permissible to feed matza on Erev Pesaḥ to small children who do not understand this.
The prohibition begins at dawn on the 14th of Nisan, though some choose to be stringent and refrain from eating matza from the beginning of Nisan. Others refrain from eating matza thirty days before Pesaḥ. However, the letter of the law only requires one to refrain from eating matza on the 14th of Nisan (MB 471:12).
Israeli hospitals and army bases ordinarily destroy ḥametz several days before Pesaḥ, because otherwise there is concern that ḥametz will remain in the kitchens and camps during Pesaḥ. Matza is served on the days leading up to Pesaḥ, so that the soldiers and patients have what to eat, but they must refrain from eating such matzot on Erev Pesaḥ.
The prohibition against eating matza on Erev Pesaḥ includes even small pieces of matza that have been kneaded with wine or oil. Even if such a mixture is baked, as long as the pieces are identifiable as matza, the blessing before eating them is “ha-motzi” and it is forbidden to eat them on Erev Pesaḥ. However, if after being kneaded and baked the pieces are no longer identifiable as matza, their berakha is “mezonot” and they may be eaten on Erev Pesaḥ (implied in Rema 461:2 and MB ad loc. 19-20). Some poskim are more stringent, maintaining that even if the matza is crumbled like matza meal, kneaded with oil or wine, and baked into cake or cookies so that they are no longer identifiable as matza, it is forbidden to eat them. This is because one who eats enough of them to constitute a meal (“kevi’at se’uda”) still must recite the berakha of “ha-motzi.” Thus, it is apparent that they have not yet lost the status of matza, and consequently the prohibition applies to pastries made of matza meal (Gra, Rav Kook, and Ḥazon Ish).
However, all poskim agree that it is permissible to eat matza balls on Erev Pesaḥ, because after being cooked they no longer carry the status of matza. Even if one eats enough of them to constitute a meal, their berakha is “mezonot,” because they are a cooked food, not a baked good (MB ad loc. 20). Moreover, even if one cooks a whole piece of matza the size of a kezayit or more, most poskim maintain that although its berakha is “ha-motzi,” one may eat it on Erev Pesaḥ (as explained below 14:1).
. Some authorities maintain that any matza that is not fit for the mitzva is permissible on Erev Pesaḥ; this is the opinion of Me’iri, R. Yeshaya di Trani (Rid), and Rivash §402. Others maintain that anything that has the taste of matza is forbidden on Erev Pesaḥ, even if it is unfit for the mitzva. This position can be imputed to several Rishonim who permitted only egg matza (Rabbeinu Tam in Tosafot on Pesaḥim 99b, Rosh, Mordechai, Tashbetz, and Maharsha; see above, 8:1). If one crumbles the matza and kneads it with oil or something sweet, whether he bakes it or not, there are some who maintain that as long as the berakha is “ha-motzi,” the mixture is forbidden, but if the berakha becomes “mezonot” it is permissible (SA 168:10). According to the stringent opinions, including Olat Re’iyah (vol. 2 p. 243 §22), any baked matza meal is forbidden on Erev Pesaḥ, since any products made from matza meal would still require the berakha of “ha-motzi” if eaten in the requisite quantities (they have the status of “pat ha-ba’ah be-kisnin”). If one cooked the matza meal, even if the cooked dish still requires berakha of “ha-motzi” if eaten in the requisite quantities, he may eat in on Erev Pesaḥ since its taste has changed. This is the opinion of MB 471:20 and SHT 19 ad loc. Some authorities are stringent; see below 14:1 and Yeḥaveh Da’at 1:91, n. 10. Nonetheless, all agree that one may eat less than a kezayit of cooked matza, as I wrote above.