(This should not be relied upon for practical halacha. When a question arises a Rabbi should be consulted.)
1. The Torah (Shemot 16:22) records that in the Midbar two portions of Mann fell on Fridays (see Rashi). The Gemara (Shabbos 117b) writes that based on the Pasuk we are obligated to take Lechem Mishneh on Shabbos.
2. Acharonim debate whether this is a biblical or rabbinical obligation. The Magen Avraham (618:10) indicates that it is only a rabbinic obligation, whereas the Taz (see Shar Hatzion 271:11) states that it is a Torah obligation.
3.The Rishonim rule that women are obligated in Lechem Mishneh. Rabbeinu Tam says despite the fact that it is a time bound positive Mitzva, women are obligated to observe Lechem Mishneh because they too were involved with the miracle of the double portion of Mann falling on Fridays. The Ran offers a different reason for the obligation. He believes that the Gemara (Berachot 20b) that teaches that women are obligated to recite Kiddush implies that women are obligated in all matters relating to Shabbos, including Lechem Mishnah (see Yabia Omer 6:28 for a full list of all the opinions. Harav Ovadia Yosef zt”l does note that Harav Shlomo Kluger zt”l seems to exempt women from Lechem Mishnah, however, as noted the Rishonim disagree with his view).
4. The Kitzur Shulchan Aruch (77:17) rules that even somebody who makes Kiddush on cake (as many do on Shabbos morning) must fulfill the mitzvah of Lechem Mishneh, which is achieved by making the berachah on two cakes or crackers.
However, this is not the common custom, and several poskim note that the obligation applies only to bread (see Orchos Chaim 289:5; Da’as Torah 274:1). Shut Minchas Yitzchak (3:13) adds that taking two whole cakes would be considered michzi k’yuharah (a haughty practice), and should not be performed in public (see Minchas Yitzchak for a discussion as to whether yuhara applies to an action done privately).
5. Harav Betzalel Stern zt”l (shu”t Bitzeil Hachochma 3:110) was asked whether a frozen challah may be added to a fresh challah for lechem mishnah, or is it necessary that both rolls be fresh? Harav Stern says that as long as the roll will thaw and become edible at the end of the meal one may use it as lechem mishnah. And since practically one may decide to extend the meal as long as one wants, most bread will become edible during the meal and may be used as Lechem Mishnah. This is also the view of Harav Chaim Kanievsky shlit”a (see Shu”t Rivevos Ephraim 2:115).
Similarly, Harav Simcha Bunim Cohen shlit”a (the Radiance of Shabbos page 76) reports that he was told by Harav Moshe Feinstein zt”l that one may use frozen challah as Lechem Mishnah. Although, the frozen challah is inedible, since one is able to heat it up (through permissible methods of course) it can be used as Lechem Mishnah. This is also the view of the Tzitz Eliezer and Harav Ovadia Yosef zt”l (Yabia Omer 8:32). Harav Yosef zt”l does add that it is preferable to borrow a fresh roll of challah.
However, Harav Shlomo Zalman Auerbach zt”l (Shemiras Shabbos K’Hilchasa page 196) feels that frozen inedible challah may not be used as Lechem Mishnah. He adds that even if the challah will be able to thaw and become edible during the meal it is possible that we require you to wait until the challah is edible and only then can you use it for Lechem Mishnah.
6. One definitely should remove the lechem mishnah from any plastic bags before reciting the blessing (opinion of Harav Sheinberg zt”l in Radiance of Shabbos page 76). The opinion of Harav Ephraim Greenblatt zt”l in Rivevos Ephraim 1:201 is that if the blessing was recited when it was in the bag you have still fulfilled your obligation.
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