If your next-door neighbors have actually ever revealed, "We go to the lake house every year the week after Memorial Day!" they may be on a fixed-week timeshare. Obviously, if you want to try a different week of the year, you're up a creek. Altering your allocated week could take an act of Congress (or a minimum of a hefty upgrade fee). The drifting week choice enables you to select your week within particular limits. The deal would be something like, "You can schedule any week between…