This gets a bit complicated and a lot of it depends on what you're trying to do. If you just want to group by week, you can do something like the following:
If DatePart('w', {table.date_field}) in 1 to 3 then DatePart('ww',{table.date_field}) - 1
else DatePart('ww', {tabloe.date_field})
The 'w' format in DatePart gets you the day of the week and the 'ww' format gets you the week number in the year.
-Dell