The Rambam (Rabbi Moshe Maimonides, Spain-Egypt, 1135-1204), in Hilchot Hametz U’masa (6:12), codifies the prohibition against eating Masa on Ereb Pesah. He goes so far as to say that one who violates this prohibition and eats Masa on Ereb Pesah is punished with Malkot (lashes).
The commentators, in explaining this Halacha, cite the comment of the Talmud Yerushalmi that "one who eats Masa on Ereb Pesah is like one who has relations with his fiancé in his father-in-law’s house." Eating Masa on Ereb Pesah is forbidden because one must wait until the night of the Seder, when the Torah requires us to eat Masa. Just as a groom must wait until after his wedding to consummate the marriage, similarly, we must wait until the night of the Seder to eat Masa, and eating Masa on Ereb Pesah is forbidden.
The question arises as to whether this prohibition applies to all Masa, or only to Masa that is suitable for the Misva on the night of the Seder. One fulfills the obligation of Masa on Pesah night only with Masa that was baked specifically for the purpose of the Misva. Often, stores sell Masa that was not baked with this intention. This Masa is permissible for eating on Pesah, but it cannot be used for the Misva of eating Masa at the Seder. Is this Masa permissible on Ereb Pesah?
Rav Yosef Shalom Elyashiv (contemporary) cites the view of three Rishonim (Medieval Halachic authorities) – the Maharam Halawa, the Tashbetz, and Maharam Benet – that one may not eat any Masa on Ereb Pesah. According to this view, as long as the Masa has the standard taste of Masa, it is forbidden on Ereb Pesah. The Tashbetz, for example, writes, "This Masa which is forbidden – it is not only Masa that was guarded [for the sake of the Misva], but rather all Masa is forbidden, except Masa Ashira [‘egg Masa’]."
Thus, one should not eat any Masa on Ereb Pesah, except egg Masa, which does not have the standard taste of Masa.
Rav Elyashiv makes an exception in certain extenuating circumstances, when it might be difficult to find other food to eat. In the Israeli army, for example, it would be permissible in the morning of Ereb Pesah to feed the soldiers Masa that is not suitable for the Misva at the night of the Seder, in accordance with the lenient view. Since there is also an opinion that the prohibition against eating Masa on Ereb Pesah begins only at midday, when it becomes forbidden to eat Hametz on the level of Torah law, in situations such as the army it would be allowed to eat in the morning Masa that cannot be used for the Misva.
According to some authorities, the prohibition against eating Masa on Ereb Pesah takes effect already at nightfall on the eve of Ereb Pesah. Rav Elyashiv rules that in deference to this view, one should not eat Masa even on the night of Ereb Pesah, but one may eat Masa that cannot be used for the Misva on the night of Ereb Pesah. Furthermore, one may eat baked products made from Masa meal – such as cakes and cookies – on the night of Ereb Pesah, but not during the day of Ereb Pesah.
Summary: It is forbidden to eat any Masa or baked Masa meal products on Ereb Pesah, except egg Masa. On the night of Ereb Pesah, it is permissible to eat baked Masa meal products, and even Masa that is not suitable for the Misva of Masa at the Seder (such as if it was not baked specifically for the Misva).
