I know we all hate the MS Outlook style of referencing dates by giving the week number of the year. I always struggle if someone gives me week number and expects me to know when that’s gonna be. I hardly know any mail client other than MS Outlook that displays weeks numbers at all.
But for peace sake I’ll show you how to display this secret information on your shiny MacOS X as well.
The trick is to create a custom format for the full data listing in the System Preferences. Just add the place holder for the Week to the format definition and it will show up in your menu bar clock and even in iCal.
Please find an excellent instruction on that on Marius van Witzenburgs Blog post