軍隊裡走出營區的一天
這是字面上的那一天：部隊離開營區，整天在野地上演練。核心畫面在這裡還沒有任何比喻，field 就是真的野地，day 就是真的一整天。
noun · C1
A day when soldiers leave the base and spend the whole day out in open country, practising how to move and work together as a unit.
野外操演日
核心畫面
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…更多收起
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, entry “field day” (kaikki extract supplied in this packet; the source itself marks the derivation with “Some postulate”).
Four branches
學校走出教室的一天
同一個畫面搬到學校：那天不上課，全校到操場上，從早比到晚。換了地點和人，骨架沒變。
noun · B1
At an American school, a day when lessons stop and the students spend the day outside running races and playing sports, often with parents there to watch.
校園運動會日
盡情發揮的一天
從「這一天不必照平常規矩來」轉成比喻：得到一個可以放手去做的機會。
noun · B2
If people have a field day, something gives them a chance to do a lot of what they enjoy and they take it to the full — very often the chance to criticise or make fun of someone who has made a mistake.
大肆發揮的機會
全員動起來的一天
美軍內部的用法：一樣是停掉平常的事、全員做同一件事、做一整天，只是那件事變成了大掃除。
noun · C1
In the US Navy, Marine Corps and Coast Guard: a day when everyone stops their normal duties and cleans the whole ship or building from top to bottom.
全員大掃除日