title: Comparing string lists to find missing value of sequence
body: <p>I have a list of days of the week that can potentially contain holidays, like Independence Day, instead of the day name, Friday. (doing weather data parsing)  Looking for a way to compare my list to a list of days of the week and find the missing one (or more).  I came up with the following and it will work fine for my purposes as long as there are not 2 in a row, but I'd really like to find a better approach and make it more solid and be able to repurpose it later.  Might there be a standard library approach to this?</p>
  
  <pre><code>daysOfTheWeek = ["sunday", "monday", "tuesday", "wednesday", "thursday", "friday", "saturday"]
  
  days = ["NOTADAY", "saturday", "NOTADAY", "monday"]
  
  repaired = []
  for i, day in enumerate(days):
      try:
          dayIndex = daysOfTheWeek.index(day.lower())
      except ValueError:
          try:
              next = indices[i+1]
              dayIndex = next-1
          except IndexError:
              dayIndex = 6
      if dayIndex &gt; 6:
          dayIndex = 0
      repaired.append(daysOfTheWeek[dayIndex])      
  
  for r in repaired:
      print r.capitalize()
  </code></pre>