(Why) is there a custom to have round challah for Rosh HaShanah?

Question

I have noticed that the local kosher bakeries sell round challot (as opposed to their usual braided loaves) in advance of Rosh HaShanah.

Is such a custom noted in any written sources?

If yes, what is the reason given for this custom?

Answer 1

Taame Haminhagim ("Likutim" (in the back), 183), in my own free translation:

The reason for the custom that we make Passover matzas round is according to what Rabbi Saadia and Maimonides wrote, that people at one time believed in multiple gods…. To keep Jews far from such fools' beliefs, they imposed then upon themselves to make all their breads round: just as a circle has no start or end and is one even shape, so is the belief of the descendants of Abraham, Isaac, and Jacob only in the sole unique one, blessed be his name. Thus, too, the custom that we make the challahs for the sabbath and holidays from the first day of Rosh Hashana until after Hoshana Raba round specifically, because on Rosh Hashana we say malchuyos (coronations) and the main point is to believe in a sole, unique king.