"Have a field day" is a phrase used to tell someone to go and have fun doing whatever they were wanting to do. Example: Kid: "I want to go play outside!" Mom: "Sure, have a field day!" Worker: "I'd like to organize the entire office!" Boss: "Have a field day!" Friend: "I'm going to run a mile tomorrow!" Other Friend: "Have a field day!"

Have a pleasant day.

Nanren888 is right. It is always used in a negative sense. Someone has a "field day" if they are given an ideal opportunity - and free rein - to do something which they've been looking forward to doing and which had a negative effect on others. For example, when politicians are involved in a scandal, the press has a field day. I have never come across this idiom used in the straightforward sense of "have fun".

A maybe more common use of "have a field day" is quite different, even if it comes form a similar concept. I don't think I have ever heard it used in the sense described in answers below. They are right, this is the literal meaning, but I suspect rarely used this way. I hav ehowever heard it frequently in another way. . To "have a field day" means to enjoy some event or thing, but usually in a way that is negative for someone else. . He saw that previous answers did not cover the meaning and had a field day explaining what they had missed. When it became obvious that they had missed out on getting the contract because their submission was in late, he had a field day blaming the department that had been tasked with getting it in on time. Hope it helps.