happy hour vs letup

happy hour

noun
  • Any small gathering, usually in the afternoon or early evening, set aside to relax and have a drink. 

  • A time of day, usually in the afternoon or early evening, when a bar or pub offers its drinks at a discounted price. 

  • Flowers. 

letup

noun
  • A pause or period of slackening. 

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