I didn't get the exception. So Checking your profile I saw that your locale is in Germany so i tried this and got the exception.
java.time.format.DateTimeFormatter dtf =
java.time.format.DateTimeFormatter.ofPattern("EEE, dd MMM yyyy HH:mm:ss zzz",
Locale.GERMANY);
String date = "Mon, 10 Sep 2018 23:57:09 UTC";
System.out.println(dtf.parse(date));
And the shordays for German are :
Short weekdays So, Mo, Di, Mi, Do, Fr, Sa
Try with this code and I bet it will work
java.time.format.DateTimeFormatter dtf =
java.time.format.DateTimeFormatter.ofPattern("EE, dd MMM yyyy HH:mm:ss zzz");
String date = "Mo, 10 Sep 2018 23:57:09 UTC";
System.out.println(dtf.parse(date));
But for your String Date to work just use UK or US Locale by passing an argument
java.time.format.DateTimeFormatter dtf =
java.time.format.DateTimeFormatter.ofPattern("EEE, dd MMM yyyy HH:mm:ss zzz",
Locale.UK);
String date = "Mon, 10 Sep 2018 23:57:09 UTC";
System.out.println(dtf.parse(date));