The Dictionary of American Slang, Fourth Edition by Barbara Ann Kipfer, PhD. and Robert L. Chapman, Ph.D. Copyright (C) 2007 by HarperCollins Publishers. Cite This Source
Idioms and Phrases with field day
A time of great pleasure, activity, or opportunity, as in The press had a field day with this sensational murder trial. This colloquial expression, dating from the 1700s, originally referred to a day set aside for military maneuvers and exercises, and later was extended to a similar day for sports and games. Since the early 1800s it has been used more loosely.