title: Average of each day over multiple years
body: <p>Quick Q:</p>
  
  <p>Have a dataset: That is a measurment per day over several years. 
  Trying to work out the average of all the same dates. i.e the average of 01/01/1995 and 01/01/1996 and 01/01/1997 etc.</p>
  
  <p>Tried this: </p>
  
  <pre><code>z=df.groupby(df.index.day,df.index.month).mean()
  </code></pre>
  
  <p>But get:</p>
  
  <pre><code>TypeError: unhashable type: 'numpy.ndarray'
  </code></pre>
  
  <p>Cheers</p>