MySql week() method equivalent

I have a bit of a problem replacing the week() method from mysql in some of my queries. The equivalent function for week($date) would be "extract week from $date". All good until I found a mysql query where the week function is called with the second parameter like this: select week(date,3). Based on mysql site, second parameter means that : "Week 1 is the first week with more than 3 days this year(for second parameter=3)". I made a couple of tests with some dates on both mysql and postgresql and noticed that "select week(date,3)" and extract(week from date) sometime returns different values. Anyone has any ideea for a workaround on this issue??

Thanks