On Shabbat one should eat three meals. The Friday night and the Shabbat morning meals require Kiddush with wine and two challot, lechem mishneh, and the third meal, the seudah shlishit, requires lechem mishneh. The correct time for seudah shlishit is the afternoon. But what if it’s Shabbat Erev Pesach, when no chametz may be eaten after the fourth halachic hour of the day? No problem, some might think. Let’s use matzah for seudah shlishit. But that cannot be done either, because one may not eat kasher l’Pesach matzah on Erev Pesach and though it is Shabbat, it is also Erev Pesach. So if you cannot use challot or matzah for seudah shlishit, what should you do?
It depends. If you are Ashkenazi, you have the following options: You may eat the seudah shlishit meal in the morning before the fourth halachic hour of the day with challot for lechem mishneh. And this is the way some poskim recommend that it be done. First you make Kiddush and Netilat Yadayim for the Shabbat morning meal and eat a kezayit amount of challah. Then you immediately recite Birkat Hamazon, grace after meals. After a short break of about a quarter of an hour (if you are pressed for time, right after Birkat Hamazon), you wash your hands again for seudah shlishit, finish all the bread and recite Birkat Hamazon again.
Alternatively, seudah shlishit can be made in the afternoon with no bread and a small amount of cooked food or fruit. According to Rabbi Avigdor Nebenzahl of Jerusalem, one should do both in order satisfy the opinions of those authorities (Sefer Yeraiyim) who maintain that seudah shlishit with lechem mishneh is a Torah requirement and those authorities (Shulchan Aruch) who maintain that seudah shlishit must be eaten in the afternoon.
In either case, the following procedures are recommended: The Shabbat meals, except for the challot, and all utensils in which they are prepared should be kasher l’Pesach. Small challot rolls or pitta bread should be used for lechem mishneh because they can be entirely consumed. The challot should be kept under wraps, separate from the rest of the kasher l’Pesach surroundings. At the Shabbat morning meal(s), a separate disposable tablecloth should be used together with disposable plates and tableware for the challah. As soon as Kiddush and lechem mishneh for the Shabbat morning meal(s) are completed as described, the disposable tablecloth and plates should be discarded and one should remove the crumbs from one’s clothes, rinse one’s mouth and sweep the floor. Then one spreads a new Pesach tablecloth, sets the table with kasher l’Pesach tableware and continues with the kasher l’Pesach meal. If one wishes to eat other chametz food with the challot, such food should be treated the same way as the challot.
Rabbi Nebenzahl suggests that in public places, such as hotels or hospitals, where there is a danger that people who are unaware of the halachot may inadvertently violate the laws of Pesach, the following procedure may be adopted: Instead of challot, kasher l’Pesach matzot may be used for all three meals in the following way. On Friday night, one may wash and say Birkat Hamazon three times in the way described above, thereby fulfilling the mitzvah of the three meals all on Friday night.
If you are Sephradi and follow the ruling of the Mechaber, which requires seudah shlishit to be eaten Shabbat afternoon, the following alternatives are available: Egg matzah may be used for lechem mishneh, provided this is done before two-thirty in the afternoon so that kasher l’Pesach matzah will be eaten with appetite at the Seder. Alternatively, one may cook kasher l’Pesach matzah in kasher l’Pesach soup before Shabbat and when it has cooled off on Shabbat one can lift it out of the pot and use it for lechem mishneh.
Shacharit is recited early and expeditiously, so that all chametz can be eaten by the fourth halachic hour of the day. Other differences to keep in mind when Erev Pesach is on Shabbat are as follows: Bedikat chametz, searching for chametz, and bitul chametz, nullifying chametz by reciting Kol Chamirah, are performed on the night preceding the thirteenth day of Nissan. Biyur chametz is performed without reciting Kol Chamirah on Friday the thirteenth of Nissan before noon. Any chametz to be used for Shabbat should be kept separate, and if possible, Rabbi Nebenzahl suggests, it should be kept together with the chametz sold for Pesach.