The Daystart() function comes in very handy as you can calculate a new date which has an offset from the given date.
DayStart( RegisteredDate , 0 , -7/24 ) AS SummaryDate
Then you can do your sums according to SummaryDate later in the load script or in a sheet in the application.
When it comes to Sundays - does it really matter that the day ends at 1PM? Will you have values occuring between 1PM and 5PM anyway? If you do how would you consider these values - where would they belong when it comes to a SummaryDate?