title: Get date of first and last day of week knowing week number
body: <p>I need to get the date of the first and last day of the week knowing the week number.</p>
  
  <p>I get  a start date and an end date, representing the first and last day of a selected week in a given year. then I need to get the start date and end date of the same week of the previous year to do a graphical comparison of some data. </p>
  
  <p>I managed to get the week number based on the given start date and end date. Now I need to get the date of the first day and last day of the same week of the previous year. How could I do this quickest ?</p>
  
  <p>EDIT: This is how I got the week number:</p>
  
  <pre><code> private int GetWeekNumber(DateTime date)
      {
          GregorianCalendar calendar = new GregorianCalendar(GregorianCalendarTypes.USEnglish);
          return calendar.GetWeekOfYear(date, CalendarWeekRule.FirstDay, DayOfWeek.Sunday);            
      }
  </code></pre>