From Longman Dictionary of Contemporary Englishhave a field dayhave a field dayinformalCHANCE/OPPORTUNITY to have a chance to do a lot of something you want, especially the chance to criticize someone The newspapers had a field day when the trial finished. → field day
Examples from the Corpus
have a field day• In such situations, information biases have a field day...• They'd have a field day.• Politicians and the media have had a field day with the incident.• The court was agog and the journalists continued to scribble away, knowing they were about to have a field day.• The slippery, deceptive Mr Clinton will have a field day.• Well, the crackpots will have a field day with these revelations, Holmes!• Any bacteria that may be in the food will have a field day and grow.• The tabloid newspapers would have a field day. 