title: Merging two dataframe by date
body: <p>I have two dataframe, using pandas, one (df_1) is the average temperature by day of the year until some point in time (for example the average temperature for all the days of 2014 until 03/01/2014) and the other (df_2) is the average temperature by day for the last 30 years. </p>
  
  <p>What I want to do is to complete the first dataframe by the mean values by day in the second, I can't use the day of the year because of some leap years but I'm not sure this the right way. I have found a way to get the average temperature by day (<a href="https://stackoverflow.com/questions/28304881/get-the-average-year-mean-of-days-over-multiple-years-in-pandas?noredirect=1&amp;lq=1">Get the average year (mean of days over multiple years) in Pandas</a>) to get df_3. My end goal is to complete df_1 for the missing days (so 04/01/2014,...,31/12/2014)</p>
  
  <pre><code>df_1 = pd.DataFrame({
                 'Date': ['01/01/2014','02/01/2014','03/01/2014'], 'T_Avg_2014': [5,6,0.7]})
  
  df_2 = pd.DataFrame({
                 'Date': ['01/01/2009','02/01/2010','01/01/2011'], 'T_Avg': [5,-8,-7]})
  
  
  
  
  index = pd.MultiIndex.from_tuples([('1', '1'),
                                     ('1', '2'),
                                    ('1', '3'),
                                     ('2', '1')],
                                   names=['month', 'day'])
  columns = [('T_Avg')]
  df_3 = pd.DataFrame([3,4,8,10],
                   index=index,
                   columns=columns)
  </code></pre>