It can be found in the house of Billy Creel inside Megaton. It appears to be a very eloquent, yet abstract way of describing having a good time. There is also a copy on the personal terminal of Arthur Maxson, inside the Citadel.
Tweedle dumpling, tweedle dare,
Curse the younglings, if you care.
For theirs is youth, and joy, and power,
Five made one in the Lightman's tower.
And when they come, these five of sun,
The Lightman's progeny have won.
So sound the trumpets, clear and loud!
And think beyond the Eastern shroud.