episode vs happening

episode

noun
  • An incident, action, or time period standing out by itself, but more or less connected with a complete series of events. 

  • An instalment of a drama told in parts, as in a TV series. 

happening

noun
  • A spontaneous or improvised event, especially one that involves audience participation. 

  • Something that happens. 

adj
  • Trendy, up-to-the-minute. 

  • Busy, lively; vibrant, dynamic; fashionable. 

How often have the words episode and happening occurred in a corpus of books? (source: Google Ngram Viewer )