Cliff:

I don't know if it was mentioned earlier, but many retail related businesses use a 4-5-4 calendar determined by the National Retail Federation (http://www.nrf.com/modules.php?name=pages&sp_id=391). Hence the division into 4-5-4 or 4-4-5 is not arbitrary. There are many anomalies that occur over the years and how they are handled is determined by this group. Hence the only easy way to handle it is by a look-up table for organizations using the NRF calendar.

Regards