title: Pandas custom week
body: <p>I'm trying to define a custom week for a dataframe.</p>
  
  <p>I have a dataframe with timestamps.
  I've read the questions on here regarding isocalendar. While it does the job. It's not what I want.</p>
  
  <p>I'm trying to define the weeks from Friday to Thrusday.</p>
  
  <p>For example:</p>
  
  <p>Friday 2nd Jan 2015 would be the first day of the week.</p>
  
  <p>Thursday 8th Jan 2015 would be the last day of the week.</p>
  
  <p>And this would be week 1.</p>
  
  <p>Is there a way to set a custom weekday? so when I access the the <code>datetime</code> library, I get the result that I expect.</p>
  
  <p><code>df['Week_Number'] = df['Date'].dt.week</code></p>