title: KeyError in datetime pandas due to Big Sur?
body: <p>I've just upgrade Big Sur 11.0.1 and I'm facing some issue while trying to execute a notebook. Weird because it worked on Catalina. Could anyone help ?</p>
  <pre><code>import numpy as np
  import pandas as pd
  import yfinance as yf
  
  
  # In[56]:
  
  
  #parameters
  RISKY_ASSET = 'ADBE'
  START_DATE = '2017-01-01'
  END_DATE = '2020-07-31'
  
  
  # In[57]:
  
  
  #Download data:
  df = yf.download(RISKY_ASSET, start=START_DATE, end=END_DATE, adjusted=True)
  
  
  # In[58]:
  
  
  #daily return
  adj_close = df['Adj Close']
  returns = adj_close.pct_change().dropna()
  print(f'Average return: {100 * returns.mean():.2f}%')
  returns.plot(title=f'{RISKY_ASSET} returns: {START_DATE} - {END_DATE}')
  
  
  # In[59]:
  
  
  #Split the data into training and test sets:
  train = returns['2017-01-01':'2020-06-30']
  test = returns['2020-07-01':'2020-07-31']
  
  
  # In[60]:
  
  
  #parameters of the simulation:
  T = len(test)
  N = len(test)
  S_0 = adj_close[train.index[-1].date()]
  
  N_SIM = 1000
  mu = train.mean()
  sigma = train.std()
  </code></pre>
  <p>Error:</p>
  <pre><code>---------------------------------------------------------------------------
  KeyError                                  Traceback (most recent call last)
  &lt;ipython-input-7-9316c025c623&gt; in &lt;module&gt;
        2 T = len(test)
        3 N = len(test)
  ----&gt; 4 S_0 = adj_close[train.index[-1].date()]
        5 N_SIM = 1000
        6 mu = train.mean()
  
  ~/opt/anaconda3/lib/python3.8/site-packages/pandas/core/series.py in __getitem__(self, key)
      880 
      881         elif key_is_scalar:
  --&gt; 882             return self._get_value(key)
      883 
      884         if is_hashable(key):
  
  ~/opt/anaconda3/lib/python3.8/site-packages/pandas/core/series.py in _get_value(self, label, takeable)
      987 
      988         # Similar to Index.get_value, but we do not fall back to positional
  --&gt; 989         loc = self.index.get_loc(label)
      990         return self.index._get_values_for_loc(self, loc, label)
      991 
  
  ~/opt/anaconda3/lib/python3.8/site-packages/pandas/core/indexes/datetimes.py in get_loc(self, key, method, tolerance)
      620         else:
      621             # unrecognized type
  --&gt; 622             raise KeyError(key)
      623 
      624         try:
  
  KeyError: datetime.date(2019, 12, 31) ```
  </code></pre>