This should solve your problem -
UPDATED With string DateTime conversion
DateTime dt = DateTime.ParseExact("28/02/2014","dd/MM/yyyy", CultureInfo.InvariantCulture);
var cal = System.Globalization.DateTimeFormatInfo.CurrentInfo.Calendar;
cal.GetWeekOfYear(dt, System.Globalization.CalendarWeekRule.FirstDay, System.DayOfWeek.Sunday);