title: Pandas Get Day of Week from date type column
body: <p>I'm using Python 3.6 and Pandas 0.20.3.</p>
  
  <p>I have a column that I've converted to date type from datetime. All I need is the date. I have it as a derived column for ease of use. But I'm looking to do some further operations via a day of the week calculation. I can get the day of week from a datetime type but not from the date. It seems to me that this should be possible but I've tried multiple variations and not found success.</p>
  
  <p>Here is an example:</p>
  
  <pre><code>import numpy as np
  import pandas as pd
  df = pd.DataFrame({'date':['2017-5-16','2017-5-17']})
  df['trade_date']=pd.to_datetime(df['date'])
  </code></pre>
  
  <p>I can get the day of the week from the datetime column 'trade_date'.</p>
  
  <pre><code>df['dow']=df['trade_date'].dt.dayofweek
  df
      date    trade_date  dow
  0   2017-5-16   2017-05-16  1
  1   2017-5-17   2017-05-17  2
  </code></pre>
  
  <p>But if I have a date, rather than a datetime, no dice:
  For instance:</p>
  
  <pre><code>df['trade_date_2']=pd.to_datetime(df['date']).dt.date
  </code></pre>
  
  <p>And then:</p>
  
  <pre><code>df['dow_2']=df['trade_date_2'].dt.dayofweek
  </code></pre>
  
  <p>I get (at the end):</p>
  
  <pre><code>AttributeError: Can only use .dt accessor with datetimelike values
  </code></pre>
  
  <p>I've tried various combinations of dayofweek(), weekday, weekday() which, I realize, highlight my ignorance of exactly how Pandas works. So ... any suggestions besides adding another column which is the datetime version of column trade_date?
  I'll also welcome explanations of why this is not working.</p>