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Baking Challahs on Erev RH:
One is to bake Challahs for Yom Tov which will be used for Lechem Mishneh and is not to buy them at the bakery as is done during the week. This matter of baking one’s own Challah is included in the honoring of Yom Tov. One is not to divert from this custom.
How much is one to bake? One is to bake at least the amount that requires one to remove Challah from the dough.
 242/12; Rama 242/1; M”E 581/51; Kaf Hachaim 581/100
 The reason: The reason behind this custom of baking Challahs on Erev Shabbos is due to the Mitzvah to honor Yom Tov. [Admur ibid] The effort exerted in the kneading and baking for the sake of Yom Tov, rather than buying it from a store, honors Yom Tov. [M”B 242/6] Likewise having fresh and hot home baked bread also honors Yom Tov. [Siddur Yaaveyz; Likkutei Mahrich]
 See also Shach Yoreh Deah 324/25 that the women are scrupulous to specifically bake enough to separate Shiur Challah on Erev Shabbos.