title: Week number of the month
body: <p>Does pandas (python) offer a way to easily get the current week of the month (1:4) from a date series? </p>
  
  <pre><code>data = {'date': ['2014-05-01', '2014-05-01', '2014-05-02', '2014-05-02', '2014-05-02', '2014-05-02', '2014-05-03', '2014-05-03', '2014-05-04', '2014-05-04']}
  df = pd.DataFrame(data, columns = ['date'])
  df['date']=pd.to_datetime(df['date'])
  
  def week_of_month(dt):
      """ Returns the week of the month for the specified date.
      """
      first_day = dt.replace(day=1)
      dom = dt.day
      adjusted_dom = dom + first_day.weekday()
      return int(ceil(adjusted_dom/7.0))
  
  
  df ['week']=np.nan
  for i in range(len(df)):
      df ['week'][i] =  week_of_month(df.date.iloc[i])
  </code></pre>
  
  <p>with a short data set it works but with a large data set takes to much resources</p>