The first week of a year is:
<ul type="square"> [*]the week with January, 1st (USA, Excel) [*]the week with at least 4 days of the new year (DIN 1355 / ISO 8601) [*]the first complete week in the year (rarely used) [/list]
I think, that SILK is using the last way to calculate the week. In our calendar, that uses ISO 8601, today we have week 49. SILK calculates week 48...