In addition to nailing down whether weeks begin on Sunday or Monday, there is also the issue of which week is the Nth week, dependant on when the company's year begins.

For example, with my previous company, the fiscal year ran from February through January (instead of January through December). To make things even more convoluted, the company followed the 4-5-4 calendar, so the "first" week of a given 'corporate' month may actually fall within the previous calendar month, or a few days into the calendar month (as defined by the way the rest of the Western world works). For example, the first week of April might actually start on Monday the 8th.

The reason for this kind of tinkering with the corporate calendar was so that it was easier to match up figures from business on a particular week this year, with business on a particular week last year. Leap years caused all sorts of havoc, and resulted in even more corporate tinkering with the calendar.

I bring this up to draw attention to the fact that to fit within a corporate culture, the software design may need to provide the flexibility for custom definition of the calendar. Certanly, within my previous company's corporate culture, any software that didn't allow for custom definition of the calendar would have been essentially worthless. Sales trends, open-to-buy (for buyers), shipping cutoff dates, profitability reporting, advertising schedules, etc., all relied upon the corporate calendar, which was, in fact, subject to change on occasion.