Per the ISO standard for date and time (8601), the method for numbering weeks is described here...
...Since Jan 1, 2009 was a Thursday that makes that week, which goes Mon-Sun, Week 1. If Jan 1 were on a Fri, Sat, or Sun it would be in Week 52 (or 53) of the previous year.
Today, Sep 27, is the last day of Week 39.
2010 BMW M3 Coupe M-DCT w/Tech-EDC, BMW M Performance Exhaust, BPM Stage I Tune, Space Gray / Fox Red Extended, and a Trunk Monkey known as "Der Stig",
plus the daily driver...
2011 BMW 135i M Sport DCT, w/BMW Performance Power Kit, Exhaust, and Brake Rotors, Space Gray / Coral Red, known as "Stiggie" with an outlaw F20 "M135i" badge.