Think of one whole day lifted out of the normal routine and spent out in the open field. Nobody does the usual work that day. Everyone goes out together, and whatever is done is done in a big way, from morning until night. That feeling — a day off the usual track, used to the full — runs through every later meaning of the word.
field day 可以想成一整天被抽離平常的作息，大家一起走到野地上。那天不做平常的事。事情做得又大又徹底，從早到晚。後面的各種意思，都像是從這個「脫離日常、整天盡情投入」的畫面長出來的。
From the word’s history
Wiktionary records a postulate, not a settled fact: the idiomatic use may have grown out of the military parade-day use, because a parade day was much easier than the soldiers’ usual drilling.
Wiktionary 提到一種推測（只是推測，不是定論）：慣用語的用法可能來自軍隊的校閱日，因為那天比平常的操練輕鬆得多，聽起來像可以放鬆享受的一天。
Wiktionary, "field day"
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