- to have an opportunity to do a lot of something you want to do, especially to criticize someone The newspapers would have a field day if their affair ever became public knowledge. (Cambridge Idioms Dictionary, 2nd ed., 2006)
- to be able to do something you enjoy a great deal, esp. criticizing someone The newspapers would have a field day if his drinking was ever widely known.(Cambridge Dictionary of American Idioms, 2003)
- to have the chance to do something that you really enjoy, especially when it causes trouble for someone else
- [to have] a great time or a great deal to do, at somebody else's expense.
- to get a lot of pleasure and enjoyment from doing something — used especially to describe getting enjoyment from criticizing someone, making fun of someone, etc.