title: Mark last day of season
body: <p>I have an issue with my dataset, where I would like to mark a last day of the season(Discount, Not discount) and I am absolutely without ideas. The marking should be a new variable (values - last day, standard day)</p>
  
  <pre><code>data = {
      'start_day': [
          '2008-01-01',
          '2008-01-02',
          '2008-01-03',
          '2008-01-04', 
          '2008-01-05'
      ],
      'start_day_no': [
          1,
          2, 
          3,
          4, 
          5
      ],
      'Season': [
          'Discount',
          'Discount', 
          'No_discount', 
          'No_discount', 
          'Discount'
      ],        
  }
  frame = pd.DataFrame(data)
  </code></pre>
  
  <p>So if I look on my full dataset, I would like to mark the last day of discount season (blue) and last day of standard price (red), however with the condition that the season lasts at least for 20 days (so red start of 2016 doesn't count as a season).  </p>
  
  <p><a href="https://i.stack.imgur.com/RzpHh.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/RzpHh.png" alt="enter image description here"></a></p>
  
  <p>Any hints, please? Thank you!!!!</p>