It came up in Germany, for only slightly obvious reasons, that no one actually knew what the "D" in "D-Day" stands for. Destroy? Demand? Deliberate?

As it turns out, it stands for nothing in particular, or, to quote the Defense Technical Information Center it refers to the unnamed day on which a particular operation commences or is to commence.

But that really sounds much better when surrounded by its cousins: C-Day, M-Day, W-Day. H-Hour. L-Hour. All of this on Zulu time, I'm not making it up.