Here is a challah trick that is applicable all across the sweet bread sphere: the moister, wetter and generally more difficult and sticky the bread dough, the more tender and cakey the bread will turn out. For challah and other egg and sugar-enriched doughs, this is exactly what you want.
My rule of thumb - only add enough flour for the dough to hold its shape. It should be a little sticky, but not unworkable. You should be able to knead it without it gluing itself to the counter (and your hands), but it shouldn't be bouncy and perfectly smooth, a little ragged around the edges is a good thing here. Add as little flour as you can bear and your challah will be all the softer for it.
Hope this helps and happy challah baking! And Happy New Year.
Oh and lest I forget, I have a recipe on Gilt Taste this week that is made for Rosh Hashanah - a honey glazed duck with apple and potato confit, so so good and I promise, it's really easy, not much hearder than roasting a chicken and so much more festive.