using System;
using System.Globalization;

public class Test
{
 public static string GetPreviousWeek()
 {
	// Gets the Calendar instance associated with a CultureInfo.
	CultureInfo myCI = CultureInfo.CurrentCulture;
	Calendar myCal = myCI.Calendar;
	
	// Gets the DTFI properties required by GetWeekOfYear.
	CalendarWeekRule myCWR = myCI.DateTimeFormat.CalendarWeekRule;
	DayOfWeek myFirstDOW = myCI.DateTimeFormat.FirstDayOfWeek;
	
	return myCal.GetWeekOfYear(DateTime.Now.Subtract(new TimeSpan(7,0,0,0,0)), myCWR, myFirstDOW).ToString();
 }
}