I decided the easiest thing would be to talk about what isn't there.
I think the best place to start is by reviewing the overall map of D.C. The streets are laid out in a grid, and we have letter streets running east-west while numbered streets run north-south. State streets run diagonally.
The likeliest answer posits that when Pierre L'Enfant was designing the city, he didn't want to include an X Street because X was a common symbol for Christ. We'll never know if that's the real reason, but it certainly sounds plausible to me.
With that in mind, let us turn our attention to the National Christmas Tree (or X-Mas Tree, if you like).
Located in the Ellipse, the tree is decorated to the nines every year, along with additional, smaller trees for each of the 50 states. It's quite a display, and the pictures don't really do it justice.
|Here it is in 2011...|
|And here it is in 2012.|