Disregard that first formula, it does not work under any circumstances (unless you round, but it will not always work). I was typing it out of memory while sitting here with a head cold (poor excuse).
Try this formula:
(Day({Table.Date}) + (7 - DayOfWeek({Table.Date}) + DayOfWeek(Date(Year({Table.Date}), Month({Table.Date}), 1)) - 1)) / 7
Again, this will only work if your week starts on Sunday. If it works on a different day, some tweaking will be needed.