title: Get Week number from a given date when week start on Sunday
body: <p>How to get the correct week number of the year from a given date when my first day of week is Sunday. </p>
  
  <p>For example:</p>
  
  <p><code>6 May 2018 - 12 May 2018</code> is week 18</p>
  
  <p><code>13 May 2018 - 19 May 2018</code> is week 19</p>
  
  <p>I'm trying the following extension method but unable to achieve the desired result. It's giving week <strong>19</strong> and <strong>20</strong> where I expect week <strong>18</strong> and <strong>19</strong> respectively.</p>
  
  <pre><code>public static int GetWeekOfYear(this DateTime date)
  {
      return CultureInfo.InvariantCulture.Calendar.GetWeekOfYear(
        date, 
        CalendarWeekRule.FirstFourDayWeek, 
        DayOfWeek.Sunday);
  }
  </code></pre>