From this ancient and well-respected thread:

The trouble with writing an epic, I find, is knowing just where to begin. So here I am, quill and parchment at the ready, a full bowl of pipeweed and, dash it, have great difficulty in beginning! That's the trouble with epics, as I suspect old Treebeard himself would say, and wasn't he a one for insisting that every story begin at the very beginning - of time, that is, and it takes all one's memories of school training to be polite to the old boy when you're rushing to catch an Eagle.

I brought this up with Gandalf when he dropped by yesterday. "Gandalf", I said, "Do you remember that old ROP we dropped into the crack of Mount Doom?" He did, of course. It was one of those rectangular - no, I mean rhetorical - questions. How could one forget? It was a tale to freeze thy blood, to make one's hair stand on end like quills upon the fretful porpentine - though I've never understood why one says porpentine when you mean porcupine. Something to do with elves, no doubt. I had been thinking of making a start by putting one of the elven marching songs on the title page, but all I can remember os 'Ding, dong, ding, dong, ding, dong, I hurry along', which would never do. Elrond would never approve.

So Gandalf applied himself to the task at hand - and that's a sight to see that makes strong men gasp and the ladies swoon. You could see the blood whizzing through that magnificent brain of his, chock full of all that health food he grazes on with Tom Bombadil. When there's a problem to be solved, just slip a few nuts and berries to old Gandalf and stand back, I say. Frightening, really.

So after a good think, Gandalf suggested Bilbo's eleventy-first birthday, and I knew right away I held the winning ticket, cash for life with no taxation. "Perfect" I told him. "That's just precisely where I'll set the starting post. You have hit the n. right on the h."