Excel has a WEEKNUM function for calculating the week number for a given date, but the results it gives depend on whether you consider that the week starts on a Monday or a Sunday. (The Excel default is to assume Sunday.) If you’re in the Monday camp, then this is actually week 27.
As the Microsoft Excel blog points out, the actual ISO standard for week numbering is substantially different again, counting Week 1 of the year as beginning on the first Monday of a week which has a Thursday in it, rather than just on January 1. And there’s another method of week counting which ignores weekdays altogether (week 1 begins on January 1, week 2 on January 8 and so forth, with an inevitable short week 53 at the end of the year).
Fortunately, automation means we don’t have to worry about this stuff too much (provided we’ve agreed with our colleagues which system to use). Hit the post for a handy spreadsheet that lets you print out a week-numbered calendar for any year using any of the four approaches, and formulas to use if you want to take the ISO approach instead.
Week Numbers in Excel [Microsoft Excel Blog]