Grouping by financial week number

Hello,

 I have a custom function to get the week number of the year, which workes fine for last year but this year our week 1 is 6th Jan - 12th Jan. Any ideas how I can modify it to recognise week 53 as 30th Dec - 5th Jan rather than 30th - 31st Dec?

="Week" + format(Datepart(Dateinterval.weekofyear,Fields!JournalCreatedDate.Value)).ToString()

Thanks

Suzi