On the day of Erev Pesach (which is the 14th of Nissan), one may not eat Matzah so that one will be able to eat Matzah that night at the Seder with appetite. One may, however, eat Matzah on the night of the 14th of Nissan (meaning the night before the Seder night, this year on Thursday night).
“Rich Matzah” made from flour which is Kosher for Pesach and mixed with fruit juice or wine, with no water added at all, may be eaten on the 14th of Nissan, since one does not fulfill one’s obligation on the night of Pesach with this kind of Matzah since the Torah states, “Bread of affliction” (Pesachim 36a); therefore, our Sages did not forbid eating this kind of Matzah. However, cake made out of Matzah Meal (Matzah that has already been baked and then ground back into a fine powder) with honey or wine mixed into it should not be eaten on Erev Pesach, for since it was already baked, it can no longer be considered “Rich Matzah.” Cooked Matzah or Matzah fried in oil may be eaten on Erev Pesach.
Beginning from the tenth halachic hour of the day (which is three halachic hours before nightfall, in New York at approximately 4:00 PM) on Erev Pesach, one may not eat a meal even if it does not consist of Matzah, for if one eats a meal from this point on, one would become full and incapable of eating Matzah at night with a hearty appetite. One may eat fruits and vegetables or a dish of rice even after the tenth halachic hour of the day, as long as one does not completely fill himself up with them.
The Matzah one uses on Pesach night to fulfill one’s obligation must be Matzah that was “guarded from the time it was harvested.” This is what is called “Shemura Matzah,” meaning Matzah produced from wheat that was guarded from coming into contact with water from the time it was harvested. It is especially preferable that this Matzah should be hand-made. Since many serious questions regarding the Kashrut of the Matzah can easily arise, one should be careful to purchase only Matzot that were baked under the supervision of a reliable Kashrut organization. Nowadays, thank G-d, hand-made (round) Matzot that are made especially for the Seder under strict rabbinical supervision are readily available, and one should try to purchase such Matzot for the Seder night.
The blessing of “Asher Kideshanu BeMitzvotav Vetzivanu Al Achilat Matzah” should only be recited on the Seder night (“nights” in the United States), however, on the remaining days of Pesach, there is no obligation to eat Matzah; thus, this blessing should not be recited.
The Responsa Ve’Heshiv Moshe (Orach Chaim, Chapter 28) deals with a scenario where a person accidentally ate Matzah on Erev Pesach. Should one recite Birkat Hamazon, for we have an important rule that one who eats a forbidden food does not recite a blessing before or after eating, as per the ruling of the Rambam (Hilchot Berachot, Chapter 1, Halacha 19) and Maran Ha’Shulchan Aruch (Chapter 196)? It would seem that the same applies to one who ate Matzah on Erev Pesach, for since he has eaten a forbidden food, he should not recite Birkat Hamazon.
Maran Rabbeinu Ovadia Yosef zt”l brings a proof that one must in fact recite Birkat Hamazon based on the law that one who is ill and needs to eat on Yom Kippur must recite Birkat Hamazon after eating. Although regarding other prohibitions such as an individual who is ill needing to eat non-kosher meat, one would not recite a blessing at all, before or after eating, there is a distinction between whether the food is innately forbidden, in which case no blessing would be recited at all, and when the food itself is permissible for consumption and there is just a time-constraint prohibiting its consumption, such as Yom Kippur or Matzah on Erev Pesach, in which case a blessing must be recited before and after eating.
Therefore, halachically speaking, one who mistakenly ate Matzah on Erev Pesach must recite Birkat Hamazon.