title: Get the first day of the week for a Pandas series
body: <p>I have the following df :</p>
  
  <pre><code>import pandas as pd
  from datetime import datetime, timedelta
  
  df = pd.DataFrame([
          ["A", "2018-08-03"],
          ["B", "2018-08-20"]
  ])
  df.columns = ["Item", "Date"]
  </code></pre>
  
  <p>I want to get the first day of the week for every line of my df. I tried to do this :</p>
  
  <pre><code>df['Date'] =  pd.to_datetime(df['Date'], format='%Y-%m-%d')
  df["Day_of_Week"] = df.Date.dt.weekday
  
  df["First_day_of_the_week"] = df.Date - timedelta(days=df.Day_of_Week)
  </code></pre>
  
  <p>But I got that error message : </p>
  
  <pre><code>TypeError: unsupported type for timedelta days component: Series
  </code></pre>
  
  <p>How can I get the first day of the week for a Series ?
  My expected result is that :</p>
  
  <ul>
  <li>"A", "2018-08-03", "2018-07-30"  </li>
  <li>"B", "2018-08-20", "2018-08-20"</li>
  </ul>