title: DateTime.ParseExact weekdays as text throws FormatException
body: <p>Why does the last line using DateTime.ParseExact throw a FormatException explaining that the provided string (datetimeAsString) is not valid because the weekday is faulty?</p>
  
  <pre><code>var format = "yyyy/M/ddd";
  var provider = new CultureInfo("en-GB"); // Have tried with several other cultures including Invariant
  var datetime = new DateTime(4444, 3, 2, 1, 5, 6, 777);
  var datetimeAsString = datetime.ToString(format, provider);
  
  // Here datetimeAsString == "4444/3/Wed"
  var x = DateTime.ParseExact(datetimeAsString, format, provider);
  </code></pre>