Let the ham sit at room temperature for 1 hour before baking. This will allow it to cook evenly.
Preheat the oven to 325 degrees F.
Make the glaze: In a medium pot over medium-high heat, combine the brown sugar, mustard, vinegar, cinnamon, paprika, chili powder, orange zest and juice, garlic and some salt and pepper. Bring to a gentle boil and cook for 10 minutes, stirring occasionally.
Place a rack in a roasting pan. Put the ham on the rack large open-face down (so the spiral cuts are stacked on top of one another). Brush half of the glaze over the surface of the ham, getting it between the slices as much as you can.
Pour about an inch of water in the base of the roasting pan; the water should not touch the ham. Tent the pan loosely with aluminum foil and bake for about 10 minutes per pound (1 hour 10 minutes to 1 hour 25 minutes).
Take the ham out of the oven and remove the foil. Raise the oven temperature to 375 degrees F. Brush half of the remaining glaze on the ham. When the oven is up to temperature, return the ham, uncovered, and bake for 15 minutes. Brush the remaining glaze over the ham and cook for another 15 minutes.
Carefully transfer the ham to a platter. Serve right away, or cover with foil and take it to your gathering!