title: How to convert Pandas Index to month name
body: <p>I am trying to change my Date index in the following time series to the month name.</p>
  
  <pre><code>website = dfFinal.groupby(['Date','Website'])
  
  websiteGroup = website['Visits'].aggregate(np.sum).unstack()
  
  Website              A        B          C
  Date                                      
  2015-01-01       18185   805769        NaN
  2015-02-01       73236   944458        NaN
  2015-03-01      101737  1003966        NaN
  2015-04-01      101018   861229        NaN
  2015-05-01       77724   845223        NaN
  2015-06-01      111503   966043        NaN
  2015-07-01      115413   937184        NaN
  2015-08-01      115215   890457       1649
  </code></pre>
  
  <p>for example I want it to look like this:</p>
  
  <pre><code>    Website           A        B          C
      Date                                      
      January       18185   805769        NaN
      February      73236   944458        NaN
      March        101737  1003966        NaN
      April        101018   861229        NaN
      May           77724   845223        NaN
      June         111503   966043        NaN
      July         115413   937184        NaN
      August       115215   890457       1649
  </code></pre>
  
  <p>I want to be able to this so my plot ticks will be the month name instead of the datetime.</p>
  
  <p>Thanks</p>
  
  <p>edit//</p>
  
  <p>same scenario but solution does not work on it:</p>
  
  <pre><code>systemType = dfFinal.groupby(['Date','Website','Type'])
  systemGroup = systemType['Visits'].aggregate(np.sum)
  systemGroup = systemGroup.groupby(level=[0,1]).apply(lambda x: 100*x/float(x.sum())).unstack()
  
  Type                      Other  Windows Mobile  Windows PC  
  Date       Website                                           
  2015-01-01 A           0.637888        0.005499   48.814957  
             B           0.686549        0.016506   54.176073  
  2015-02-01 A           0.742804        0.020482   49.811568  
             B           0.651802        0.014506   57.014288  
  2015-03-01 A           0.668390        0.014744   50.087972  
             B           0.573924        0.015937   59.906013  
  2015-04-01 A           0.662258        0.015839   49.310024  
             B           0.583933        0.013469   59.490449  
  2015-05-01 A           0.666461        0.020586   48.522979  
             B           0.577954        0.017983   58.838200  
  
  
  systemGroup = systemGroup.rename(index=lambda x: x.strftime('%B'))
  </code></pre>
  
  <p>gives me an error</p>
  
  <pre><code>AttributeError: 'str' object has no attribute 'strftime'
  </code></pre>