Unable to find documentation for 'BYDAY' parameter

In the given example, recurrenceRule: 'FREQ=WEEKLY;INTERVAL=1;BYDAY=MO,WED,FRI;UNTIL=20231225', here the Monday is mentioned in 2 letters as 'MO' while Wednesday is mentioned as 'WED'. Unable to find conclusive documentation which stats that we are suppose to use 3 letter or 2 letter format for passing days. Thank you for such a good library. 