The earliest written source for this custom is the sefer Ohev Yisrael by Rabbi Avraham Yehoshua Heshel, written in the 1800s. He calls schlissel challah "an ancient custom," and offers several kabbalistic interpretations.

When was the challah invented?

It wasn't until the late 1400s that Shabbos loaves are described as “challah” in a Judaic text. That text was the Leket Yosher, written by Rav Yosef ben Moshe, a Bavarian student of Rav Yisrael Isserlein, widely known as a leading rabbinical authority of his time.

What is the symbolism of challah bread?

According to Wilhelm, using challah to mark a sacred time is actually something that happens every Friday night when Jews celebrate the end of the week and the coming day of rest with Shabbat. The author's braided round challah.