Answer:
Only two backbones and thousands of ribs.
I grow for a surface, even if you cut me. I continue to grow even after death.
Held firmly in the hands, like a sword it cuts deep. Bloodless strokes, all, then forward we leap.
What month of the year has 28 days?
It comes only before, it comes only after. Rises only in darkness, but rises only in light. It is always the same, but is yet always different.
Black we are and much admired. Many seek us if they are tired. We tire the horse, and comfort man, and turn White when we've fulfilled your plan.