title: Adding day of the week column according to multiple columns in python
body: <p>How to add the <code>day_of_week</code> column(the day of the week, eg. 1 = Mon, 2 = Tue) to <code>df1</code> according to the <code>year</code>,<code>month</code>,<code>day</code> values as shown below:</p>
  
  <pre><code>    year month day  A       B       C   D       day_of_week
  0   2019    1   1   26.2    20.2    0.0 32.4        2
  1   2019    1   2   22.9    20.3    0.0 10.0        3
  2   2019    1   3   24.8    18.4    0.0 28.8        4
  3   2019    1   4   26.6    18.3    0.0 33.5        5
  4   2019    1   5   28.3    20.9    0.0 33.4        6
  </code></pre>