title: Converting date to Monday date of each week
body: <p>I have a <code>df</code>:</p>
  <pre><code>date
  2021-06-28
  2021-06-29
  2021-06-30
  2021-07-02
  2021-07-04
  2021-07-07
  2021-08-06
  2021-08-07
  </code></pre>
  <p>I am trying to convert this to week, I know I can use <code>df.date.dt.isocalendar().week</code> but this returns the week number with the default start date, whereas my start date is <code>2021-06-28</code>.
  I can achieve the dates in <code>df</code> using <code>pd.Grouper(key = 'date', freq = 'W-MON', label = 'left', closed = 'left')</code> in a <code>groupby</code> but I would like to have the same output without using <code>groupby</code> - simply applying a function to <code>date</code> column.</p>
  <p>My goal is to have:</p>
  <pre><code>date
  2021-06-28
  2021-06-28
  2021-06-28
  2021-06-28
  2021-06-28
  2021-07-05
  2021-08-02
  2021-08-02
  </code></pre>
  <p>Because these dates represent the Monday of each week that the date falls in.</p>