title: pandas datetime set Sunday as first day of the week
body: <p>I've got a dataframe of pandas with a series of dates (all Sundays) like this:</p>
  
  <pre><code>Date        Year    Week
  2011-01-02  2011    52
  2011-01-23  2011    3
  2011-01-23  2011    3
  2011-01-30  2011    4
  2011-01-30  2011    4
  </code></pre>
  
  <p>The week is given by <code>df['Date'].dt.week</code>, and what I want is set Sundays as the first day of the week, so I can get:</p>
  
  <pre><code>Date        Year    Week
  2011-01-02  2011    1
  2011-01-23  2011    4
  2011-01-23  2011    4
  2011-01-30  2011    5
  2011-01-30  2011    5
  </code></pre>
  
  <p>How can I do that in the simplest way?</p>
  
  <p>P.S.    I have failed to mention that there're multiple years in this dataset. So for rare occasions there'll be the last day of the year is Sunday, I would like to get <strong>The 53rd week of this year</strong> other than <strong>The 1st week of next year</strong>.</p>