The "weekly" rate is simply 7 consectutive nights of that "season" added together for convenience. The point total is the same. There is no "discount" for staying a week.
As Chuck indicated, the weekly rate is published merely for convenience and is equal to 5 week nights (Sun-Thur) plus 2 weekend nights (Fri-Sat) for that villa for a particular season.
One thing that people are sometimes unaware is that the nightly point total can change if your vacation spans two "seasons". The point per night calculation is made based on what season that night falls in.
Yes be sure to look at the seasons your dates fall in. I'm staying at OKW end of Sept/ begining of Oct for 1 week and it is two different seasons. So my "weekly" rate isn't actually listed in the book. I hope that made sense!