Obtener el Numero de la Semana

[Peterpay]

DateTime date = new DateTime(2003,12,31);
System.Globalization.CultureInfo norwCulture =
System.Globalization.CultureInfo.CreateSpecificCul ture("es");
System.Globalization.Calendar cal = norwCulture.Calendar;
int weekNo = cal.GetWeekOfYear(date,
norwCulture.DateTimeFormat.CalendarWeekRule,
norwCulture.DateTimeFormat.FirstDayOfWeek);
// Show the result
this.textBox1.Text = weekNo.ToString();

algo asi funciona