title: Week of a month pandas
body: <p>I'm trying to get week on a month, some months might have four weeks some might have five.
  For each date i would like to know to which week does it belongs to.  I'm mostly interested in the last week of the month.</p>
  
  <pre><code>data = pd.DataFrame(pd.date_range(' 1/ 1/ 2000', periods = 100, freq ='D'))
  
  0  2000-01-01
  1  2000-01-02
  2  2000-01-03
  3  2000-01-04
  4  2000-01-05
  5  2000-01-06
  6  2000-01-07
  </code></pre>