After The Lord Mayor's Show Meaning

(idiomatic) Said of a disappointing or mundane event occurring straight after an exciting, magnificent, or triumphal event.

Example: 1995 Dec. 10, Clem Thomas, "White hot Swansea", The Independent (UK):
  The All Whites came into this match hotfoot from their stunning victory over Castres last Tuesday which took them into the semi-final of the prototype European Cup. ... For a while it looked as if it might be a case of after the Lord Mayor's show as Swansea struggled with one of their dozy moods, against Treorchy.
2006, Jeremy Carrad, Running on Empty, (Pegasus Elliot Mackenzie, ISBN 1843862182), p. 40:
  "After the Lord Mayor's Show."
  Freddy looked up from reading Punch.
  "What is, old fruit?"
  "The excitement's gone. Back to dull monotony."