Siman 609

In the Tur, he brought the words of the Geonim, who forbade insulating hot food on the eve of Yom Kippur for the conclusion of Yom Kippur. The Tur wondered at them. And so too our teacher, the Beit Yosef, in his great book: What prohibition is there in this matter to prepare from a weekday for a weekday? And if it is because it remains on the stove during Yom Kippur—what of it? Behold, it also stands on the stove on Shabbat. Therefore, our teacher, the Beit Yosef, wrote: It is permitted to insulate hot food on the eve of Yom Kippur for the conclusion of Yom Kippur; thus far his words. That is to say: there is no reason for a prohibition.

Nevertheless, since the matter emerged from the mouths of the Geonim—they refrained from doing so. This is what our teacher, the Rema, wrote: And some say that one should not perform hatmanah on Yom Kippur, and such is the custom in these provinces. Thus far his words. And some say the reason is because on every Erev Shabbat it is not proper to perform work after midday, except for the honor of Shabbat, and such is the case on Erev Yom Kippur. And since this is for a weekday—it is not appropriate (Bach). And some say so that his soul shall be grieved within him (see Taz). And there is one who wrote that because on Yom Kippur the cooked food becomes heated, and this is preparation for a weekday (see Magen Avraham). And there is no reason in this reason. And perhaps it is so that the members of the household do not smell the scent of the cooked foods in the manner of hatmanah, and due to the desire for the scent they may come to a state of bulmos.