This Halacha is an excerpt from our Sefer
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It is customary for every household to bake Challahs for Yom Tov which is used for Lechem Mishneh, and 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 Shabbos and Yom Tov. One is not to divert from this custom.
 Admur 529:2; 242:12; Rama 242:1
 The reason: The reason behind this custom of baking Challahs on Erev Yom Tov is due to the Mitzvah to honor Yom Tov. [Admur ibid] The effort exerted in the kneading and baking for the sake of Shabbos, rather than buying it from a store, honors Yom Tov. [M”B 242:6] Likewise having fresh and hot home baked bread likewise honors Yom Tov. [Siddur Yaavetz; Likkutei Maharich]