I have studied the code in Calendar.php extensively the last week, in order to try and fix this problem - but it seems to be a little over my head, and despite a lot of work I didn't manage to get the week numbers to display correctly.
Obviously, SMF only supports the week number definition that the week containing January 1st is the first week of the year, and there is no option to change week number standard. But many European countries (such as the UK, Germany, Scandinavia, etc) define week number 1 as the first week containing at least four days of the current year (I e '%V' in PHPs strftime).
Unfortunately, it isn't quite as simple as just changing '%w' to '%V', as the addition of week numbers for the other weeks in the month (as the week number is only calculated once, and then +1 is used) is screwed up when it reaches the new year.
Can someone more experienced with SMF and Calendar.php please look into this problem, and make a quick fix to change week numbers to the European standard (I e the ISO-8601 standard, which is explained under '%V' in PHPs documentation for strftime)?
SMF really should add an option for the different week number definitions in the next version, but while waiting for that, most of us in Europe would appreciate a quick fix to show the correct ISO-8601 week numbers in the calendar. It can't be that hard...