Code courtesy of Bob Askew (raskew on Access World Forums)

Function GetPrevSunday(dteDate As Date) As Date
    GetPrevSunday = DateAdd("d", -Weekday(dteDate) + 1, dteDate)
End Function

Function GetNextSunday(dteDate As Date) As Date
    GetNextSunday = DateAdd("d", -Weekday(dteDate) + 8, dteDate)
End Function