Comparable thread some time ago. You have to do some calculation with DATEPART and DATEADD and stuff.
You should use DATEPART(wd, <<first of your month>>) to get the weekday of the first of the month, and use that to calculate the first sunday.
Something along the lines of
if DATEPART(wd, "12/01/2003) = 0
return "12/01/2003"
else
return DATEADD(dd, "12/01/2003", 7-DATEPART(wd, "12/01/2003"))
For the last, you do comparable arithmetic but backwards ...