Got a call from a customer the other day who could not understand that the week numbers that showed up in his Notes 8.x calendar was different from the ones on this printed wall calendar. The reason for this is that by default Notes 8 uses the operating system default for calculating week numbers. Here in Denmark we use the ISO 8601 standard which is a little bit different. It is my understanding that previous releases of Notes automatically used the ISO standard. I could be wrong though.
Thankfully the Notes developers know about these regional differences and there is an option to change the way week numbers are calculated. To change it open File/Preferences... and open the "Calendar and To Do/Regional Settings" page. On the bottom you'll find a option to change between using the operating system default and using the ISO standard. There even is a way of doing custom week numbers if that's needed.
So there you have it. For most the ISO standard is probably the way to go.