title: How to change week start date using df.resample?
body: <p>I'm using Python 3.6 and this is my code:</p>
  
  <pre><code>trainFile = r'C:\download\combine\Combine_1.csv'
  pwd = os.getcwd()
  os.chdir(os.path.dirname(trainFile))
  df = pd.read_csv(os.path.basename(trainFile))
  os.chdir(pwd)
  
  df['date'] = pd.to_datetime(df['date'])
  df = df.set_index('date')
  week = df.resample('w').mean()
  
  week.to_csv('week_combine.csv')
  </code></pre>
  
  <p>I try to make daily data into weekly data, but now the week starts at every Sunday, how can I change it into every Monday?</p>
  
  <p>For example, now the results look like:</p>
  
  <pre><code>...
  2017-09-10
  2017-09-17
  2017-09-24
  ...
  </code></pre>
  
  <p>but I want to change it into:</p>
  
  <pre><code>...
  2017-09-11
  2017-09-18
  2017-09-25
  ...
  </code></pre>
  
  <p>Any idea?</p>