title: Compute monthly averages from daily data
body: <p>I have this dataframe "df1" as example which is actually part of a much larger one (15 years):</p>
  
  <pre><code>             X1          X2
  3798 2009-12-29           0
  3799 2009-12-30           0
  3800 2009-12-31           0 
  3802 2010-01-02           0
  3803 2010-01-03         2.1
  3804 2010-01-04           0
  3805 2010-01-05           0
  3806 2010-01-06           0
  3807 2010-01-07           0
  3808 2010-01-08           0
  3809 2010-01-09           0
  3810 2010-01-10         6.8
  3811 2010-01-12           0
  3812 2010-01-13           0
  3813 2010-01-14        17.7
  3814 2010-01-16           0
  3815 2010-01-17           0
  3816 2010-01-18         1.5
  3817 2010-01-19           0
  3818 2010-01-20           0
  3819 2010-01-21           0
  3820 2010-01-22           0
  3821 2010-01-23           0
  3822 2010-01-24           0
  3823 2010-01-25           0
  3824 2010-01-26           0
  3825 2010-01-27         4.5
  3826 2010-01-28           0
  3827 2010-01-29           0
  3828 2010-01-31           0
  3829 2010-02-01           0
  3830 2010-02-03           0
  3831 2010-02-04           0
  3832 2010-02-05           0
  3833 2010-02-07           0
  3834 2010-02-08           0
  3835 2010-02-09         1.2  
  </code></pre>
  
  <p>I want to use this dataframe to create a new one "df2" with averages per month. Does anyone know how to do this? Help would be great!</p>