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 ...