title: python/pandas Pull month just once from dataframe index
body: <p>Currently I have a dataframe that looks like this below:</p>
  
  <pre><code>df = 
  
              Portfolio Value  Reference Values
  Date                                         
  2017-12-08     1.116178e+06      1.131236e+06
  2017-12-15     1.117073e+06      1.139133e+06
  2017-12-22     1.132547e+06      1.148579e+06
  2017-12-29     1.138442e+06      1.147916e+06
  2018-01-05     1.164225e+06      1.176083e+06
  2018-01-12     1.187353e+06      1.196030e+06
  2018-01-19     1.189111e+06      1.201825e+06
  2018-01-26     1.206821e+06      1.225157e+06
  2018-02-02     1.158117e+06      1.176250e+06
  2018-02-09     1.100856e+06      1.115643e+06
  2018-02-16     1.147286e+06      1.161896e+06
  2018-02-23     1.150850e+06      1.169671e+06
  2018-03-02     1.131918e+06      1.142618e+06
  2018-03-09     1.176213e+06      1.183319e+06
  2018-03-16     1.169246e+06      1.168313e+06
  2018-03-23     1.115895e+06      1.107276e+06
  2018-03-30     1.142702e+06      1.129814e+06
  2018-04-06     1.131622e+06      1.115807e+06
  2018-04-13     1.155948e+06      1.140644e+06
  2018-04-20     1.167440e+06      1.151066e+06
  2018-04-27     1.161850e+06      1.147982e+06
  2018-05-04     1.156383e+06      1.146540e+06
  2018-05-11     1.188791e+06      1.175786e+06
  </code></pre>
  
  <p>I would like to get a list of months from the dataframe with weekly data in it but not repeating the same month. For example, right now in the beginning of the dataframe month 12 shows up four times, however in my list I would like for it to only show up once. This statement is valid for all months.</p>
  
  <p>I would like the end result to look something like this:</p>
  
  <pre><code>monthList = [12, 1, 2, 3, 4, 5] 
  </code></pre>
  
  <p>my actual dataframes would have many years with varying start months and end months so I was hoping the end result would look something like this:</p>
  
  <pre><code>monthList = [12, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 1, 2, 3, 4, 5] #etc...
  </code></pre>
  
  <p>the <code>monthList</code> should start on the month in the dataframe and end on the month the dataframe ends on. </p>