Group Data By Weeks

hi, I am trying to group my data into weeks in a month.
I am using :
GROUP BY DateAdd(day, -1 * datepart(dw, convert(char(10),date1,23)),
convert(char(10),date1,23))

Which works ok, but my first day is the monday of each week.
Do you know how I can group by weeks,
where dates run from monday to sunday ?
So IN march I would have

week begin 3/3
week begin 10/3
week begin 17/3
week begin 24/3
week begin 31/3

But for the last day I only want to count the 31st, not the entire week.
Same goes for the beginning of the month,
eg for april, I want it to show as week beginning 31st march but I only count aprils data.

I hope that makes sense.

can you help ?