Today as I was walking I amused myself by singing (or trying to sing) Christmas carols. This is an eye-opening exercise. I’m one of those who sing along to most hymns—but especially Christmas carols—with only half an eye on the hymnal because “I know the words.” What this boils down to in principle is that, given the first half of any rhyming couplet in most common hymns, I can supply the rest, so I can manage to follow along without noticeably losing my place.
It’s a bit different when you’re cast out entirely on your own. Try it. Start singing any familiar Christmas carol and see how far you get. If you’ve been attending church or listening to the radio or Christmas CDs during Advent, you’ll almost certainly manage the first verse without difficulty. But what comes next? How much of these traditional carols do you really know? And can you keep the verses in order? “We Three Kings” is a fairly easy one because it has a logical structure (though I did struggle trying to come up with the wording about frankincense); others are less well organized. And you may find that some are a lot more repetitive than you realized (and not always in a good way). The syntax of “Once in Royal David’s City” defeated me; I hadn’t realized that it begins with two dependent clauses and no independent clause, so I was looking for a word that wasn’t there. And very oddly (though perhaps less oddly if you realize I was once a Latin teacher), I had to resort to the Latin words of “Adeste Fideles” to come up with the correct ones for “O Come, All Ye Faithful.”
If you find yourself sitting around with a lot of relatives on Christmas afternoon, too full of turkey or ham or whatever to budge from the sofa, you might suggest this to them as a sort of parlor game. You might even divide up into teams and brainstorm to see which team can provide the most correct verses of a given carol. You may be surprised at what you remember—and what you don’t.
WalkThoughts 
Correction: When I looked at the program for the “Festival of Nine Lessons and Carols” and saw the lyrics for “Once in Royal David’s City,” I realized I’d gotten them wrong; there is in fact an independent clause. I was remembering “Once in royal David’s city in a lowly cattle shed…” In fact, the words are “stood a lowly cattle shed.” Mea culpa.