title: Calculate maximum value by day of the year over certain period
body: <p>I have the following <strong>data frame</strong>:</p>
  
  <pre><code>my_index = ['2005-03-20', '2008-03-20', '2014-03-20', '2007-08-15', '2012-08-15', '2007-12-31', '2011-12-31', '2013-12-31', '2014-12-31']
  df = pd.DataFrame([42, 51, 36, 217, 228, -56, -50, -66, -32], index = my_index, columns = ['Temperature'])
  df.index = pd.to_datetime(df.index)
  </code></pre>
  
  <p>Which looks like this:</p>
  
  <pre><code>            Temperature
  2005-03-20           42
  2008-03-20           51
  2014-03-20           36
  2007-08-15          217
  2012-08-15          228
  2007-12-31          -56
  2011-12-31          -50
  2013-12-31          -66
  2014-12-31          -32
  </code></pre>
  
  <p>What I want is to <strong>calculate the maximum value by day of the year over the period 2005-2014</strong>. The <strong>desired output</strong> would be this one:</p>
  
  <pre><code>       Temperature
  03-20           51
  08-15          228
  12-31          -32
  </code></pre>
  
  <p>Any idea? Thx.</p>