title: Pandas Dataframe group by week
body: <p>I have a pandas Dataframe which contains a set of fixtures of sports events with over a years worth of data.</p>
  
  <p>I want to create a new column in the dataframe, 'round' which increments week to week so that I can work on a larger set of matrices (of the fixtures) round by round.</p>
  
  <pre><code>import pandas as pd
  dta = pd.read_csv(...) #read data
  indexedDta = dta.set_index(['Date']) #index the raw data.
  indexedDta['Round'] = 0 #add in the new column and give dummy value.
  indexedDta['Round']= indexedDta.groupby(by=dta.Date) #wrong
  </code></pre>