This isn't just to make Aunt Edna think you're a great housekeeper. A clean oven cooks food more evenly, and if you've ever needed an oven you can count on, this is the time. Dirty ovens will also send out plumes of foul-smelling smoke as the food stuck to the bottom of the oven continues to burn. Thanksgiving Day is not the time to call on your local fire department to come see your tarnished oven.
Self-cleaning ovens make the task easier, but even if you don't have one, you'll be ahead of the game if you expend the elbow grease to get your oven into shape now. There are severl products on the market that can help cut down on a sore arm and aching back, too.
And make sure you clean the oven far enough in advance that the oven cleaner smell has time to disappear from the air. Nothing will turn appetites off faster than a chemical odor wafting out from where the bird is cooking.