Under certain circumstances it is possible, that Lotus Notes will display the wrong calendar week. This is due to different counting of calendar weeks in e.g. Europe and U.S. (in Europe weeks starting on Monday in the US on sundays). In my case this happened, when I set my Mac OS Lion to English and thus to an American Environment. For example the last week of this year is displayed as calendar week 53 as can be seen in the following screenshot:
To fix this and have the correct week shown we must set the calendar week counting to ISO standard. To do so go to the preference pane and open Calendar and To Do followed by Regional Settings. Now you can switch the radio button on the bottom from “Use my operating system regional setting” to “always use ISO standard“.
After clicking OK switching back to the calendar show that the week number has changed to week 52: