Why do we begin eating with the bottom challa on Friday evening, and only then; not on Shabbat morning and not on holidays (unless they fall on Fri evening)?
It's Al Pi Kabbalah. See Rama 274:1. See Alter Rebbe Shulchan Aruch 274:2 who mentions that on Friday night, in order not to pass on a mitzva, one should place the lower challah closer to oneself, or during the brocha one should place the upper challah on bottom.
Also see Levush 274:1 for a small explanation on the Kabbalistic reason.