I first learned this technique of mixing challah dough from Israeli baker Uri Scheft. Uri is the author of Baking Breads and owns Lehamim in Tel Aviv and started Breads Bakery in New York. I incorporated some of his techniques into my recipe.
Braided challah is served on Shabbat and all Jewish holidays except Passover when only unleavened bread is served. On Rosh Hashanah, round challah is served because it is the most holy holiday and it represents the circle of life. It is traditional to serve it with honey to wish everyone a sweet new year. Shana Tova.