In Europe, week numbers are used pretty extensively: The board is due to meet in week 33. Michael will be gone for military service in weeks 24-25. Please submit the quarterly numbers by week 17. Did you know there are at least 6 different ways that weeks are numbered?
Most of Europe seems to follow ISO 8601 numbering, which seems to be what BSD's "ncal -w" computes. But I need to fix my quarterly agenda generator in postscript (that I hacked up starting with pscal code), since I apparently coded up the American numbering by mistake.
If you're always online, there is of course a web-based alternative for looking it up. Its not clear which week number standard yahoo's calendar uses, since I only remember being able to specify which day the week begins on. Maybe it is inferred from your configured time zone?