title: Python sort values recorded in the same calendar date across multiple years
body: <p>I am working on a dataframe that contain daily values recorded over a multi-year time span like this</p>
  
  <pre><code>   Date    Position  Value
  2010-01-01 PEAK      60
  2010-01-01 BOTTOM    51
  2010-01-02 PEAK      62
  2010-01-02 BOTTOM    50
  ...
  2011-01-01 PEAK      70
  2011-01-01 BOTTOM    61
  2011-01-02 PEAK      72
  2011-01-02 BOTTOM    60
  ...
  2015-12-31 PEAK      92
  2015-12-31 BOTTOM    83
  </code></pre>
  
  <p>That is, row entries corresponding to each calendar date are repeated for 6 years in a row, disregarding leap years (i.e., Feb. 29th) and i want to group them by calendar date regardless of years like the following format</p>
  
  <pre><code>       Date    Position  Value
      2010-01-01 PEAK      60
      2010-01-01 BOTTOM    51
      2011-01-01 PEAK      63
      2011-01-01 BOTTOM    57
      ...
      2015-01-01 PEAK      84
      2015-01-01 BOTTOM    71
      ...
      2014-12-31 PEAK      85
      2014-12-31 BOTTOM    79
      2015-12-31 PEAK      92
      2015-12-31 BOTTOM    83
  </code></pre>
  
  <p>What will be a better way to do such groupby() operation, suppose I want to maintain the leeway to do some simple calculation (e.g., max(), min(), mean()) on the values within each calendar date "group"?</p>
  
  <p>I did check all relevant previous post but cannot seem to find an appropriate for my purpose here. I will really appreciate if someone could point out a better way to do this.</p>
  
  <p>Thank you.</p>