If at all possible, try to make this recipe with nutmeg. It has that rich, warm flavor that makes this cake taste like Christmas.  If you have to substitute, you can try using mace which is a different part of the same plant that nutmeg comes from. The flavor is a bit different but probably the most similar.  If you leave it out altogether, you are basically making a cinnamon cake. Which is fine. But it won't sing as this nutmeg cake will.