An April Fool is defined as: "One who issportivelyimposed upon by others on the first day of April."
Let's look and see what the Bible says
"Speak not in the ears of a fool: for he will despise the wisdom of thy words."
"For a dream cometh through the multitude of business; and a fool's voice [is known] by multitude of words."
Let's look and see what the celebrities say
"The first of April, some do say, Is set apart for All Fools' Day. But why the people call it so, Nor I, nor they themselves do know. But on this day are people sent On purpose for pure merriment."
~Poor Robin's Almanac(1790)~
"The first of April is the day we remember what we are the other 364 days of the year."
~By Mark Twain~
Remember these words on April Fool's day
April 1st can be filled with 'giggles and smiles', it is okay to have fun...
but don't do things to "hurt" other people. Treat others as you would want them to treat you...now go and have a bit of harmless fun."
Let's learn some words and expressions
Fool: one who counterfeits folly; a professional jester or buffoon in dramas （我们常见的fool即“傻瓜”，但如果读莎士比亚的戏剧如King Lear《李尔王》，你会发现国王身旁常常有一个"Fool"。这时可要记住哦，Fool在这里可不是傻子而是绝顶聪明的“宫廷小丑”，他拿自身的滑稽或外显的愚蠢和疯癫作为护身符，说出一般人不敢或不愿说出来的真话。）
play the fool/act the foll: to act like the idiot or foolishly（胡闹，办傻事）
fool around:to spend time doing nothing useful（虚度光阴，游手好闲）
Fool's haste is no spead: 欲速则不达
make an April fool of sb: 把某人当作愚人节恶作剧的对象
fool's erand: a fruitless mission or undertaking（徒劳无用的工作）
fool's paradise:happiness founded on a illusion（虚幻的美梦，不可实现的希望）
hoax: an attempt to trick an audience into believing that something false is real;an outrageous claim that is publicly exposed as a lie or deception（根据英文释义，汉语中的“骗局”与hoax很贴切，常常指公众群体受到愚弄。在西方，媒体往往在愚人节这天制造一些出格的hoax给公众开玩笑）。
例句：I hoaxed you into believing that the mouse has eaten the cat（我骗你相信老鼠吃了猫）。
prank: a mischievous trick（恶作剧，与hoax相比，prank更侧重于朋友之间或熟人之间的玩笑或恶作剧。搞恶作剧的人常常被称作prankster）
例句：He likes playing pranks on the fellows around（他喜欢搞些恶作剧给周围的人）。
practical joke: 与“verbal joke”（耍嘴皮开玩笑）相对应，practical joke更侧重于行动。当然，既然已有古训“Action speaks louder than words”，那么practical joke一定不是小儿科式的低级玩笑，它需要搞恶作剧的人多费些心思，使玩笑更高明一点。
例句：The first day of April is devoted to practical jokes（人们喜欢在愚人节开玩笑）。
feel like a fish out of water: 犹如鱼而离水，感到生疏，不能适应环境
drink like a fish: 酗酒, 牛饮
All is fish that comes to his net: 来者不拒
Never offer to teach fish to swim: 不要班门弄斧
The best fish swim near the bottom: 珍贵的东西不能轻易得到
Never fry a fish till it's caught: 不要操之过急
April fish: 愚人节被愚弄的人，法国人愚弄他们朋友的方式就是在朋友的背后贴上一条纸做的鱼，当有人发现这个恶作剧时，他们就会大笑着喊:“Poisson d'Avril”（四月鱼）！这一说法缘于四月份的鱼刚刚孵化出来，很容易上钩被捉。