Allowは、許可する、許可される、という意味があります。
"You're only allowed to lie on April Fools' Day"
➔エイプリルフールの日だけ、嘘を付くことが許可される→エイプリルフールの日だけ、嘘をつくことができる
エイプリルフールは、April Fools' Day（エイプリルフールズデー）と言います。Foolは、単数形なので、複数形のFools、「's」は「の」と言う意味です。
Foolは名詞でバカという意味ですが、動詞として使うと、バカにする、ふざける、という意味になります。
少しでもお役に立てれば幸いです。
That lie was only suppose to be told on Aprils fools day.
Since today is the "10th May" and not "1 April (Aprils fools day)" that joke (or lie) was not funny.
▪That lie was only suppose to be told on Aprils fools day.
"only suppose to" means that it can't be said any other day but Aprils fools day.
.......,.....,.....,...
▪Since today is the "10th May" and not "1 April (Aprils fools day)" that joke (or lie) was not funny.
This is saying in a sarcastic manner that the lie was not funny because it was not said on Aprils fools day.
"sarcastic" means to say the opposite of what you want to say.
Other sentences:
a. Lies like that is only meant for Aprils fools day.
【例】
▪That lie was only suppose to be told on Aprils fools day.
（その嘘はエイプリールフールだけです）
この"only suppose to"とは、他の日ではなく4月1日（エイプリール）だけです、という意味になります。
.......,.....,.....,...
▪Since today is the "10th May" and not "1 April (Aprils fools day)" that joke (or lie) was not funny.
（今日は5月10日で4月1日/エイプリールじゃないから、その冗談は面白くありません）
これは、皮肉めいて、今日はエイプリールフールではないので、その冗談は笑えない、と言う事を言い表しています。
"sarcastic"（皮肉/嫌味）とは言いたい事の反対の事を言う事を意味します。
【その他の表現】
a. Lies like that is only meant for Aprils fools day.
（そんな嘘はエイプリールフールだけです）
You're only allowed to tell lies/jokes on April Fools Day
April Fools Day is the day for telling lies/jokes
April Fools Day is the 1st April and it is usually a day where people tell jokes or play pratical jokes (pranks) on people it is seen as a joke and to have a laugh
To tell lies is more of an intentional thing, telling jokes is more in a happy way and to have fun
April Fools Day'は4月1日です。この日は冗談を言ったりイタズラをしてみんなで笑い合います。
"To tell lies"（うそをつく）はどちらかというと意図のあるもので、"To tell jokes"はもっと楽しいものを言います。
It's not nice to tell lies except on April Fool's Day.
That's a good one. You should save it for April Fool's Day.
That would be better to tell on April Fool's Day not today.
An admonishing way is to say, "It's not nice to tell lies except on April Fool's Day".
However, if you are trying to be polite then you can say it like, "That's a good one. You should save it for April Fool's Day", or "That would be better to tell on April Fool's Day not today."
諭すような言い方は、
"It's not nice to tell lies except on April Fool's Day".（エイプリルフールでない日にうそをつくのは良くないな）
です。
ただ、もし丁寧に言いたいなら、次のように表せます。
"That's a good one. You should save it for April Fool's Day"（面白いね。エイプリルフールに取っておいた方が良いよ）
または、
"That would be better to tell on April Fool's Day not today."（それは今日じゃなくエイプリルフールに言った方が良いよ）
"That would be a good idea to say on April Fool's Day not today"
If you wanted to explain to someone that you don't believe telling lies is a good idea unless it is April Fool's Day, you can simply state this by saying "You're only allowed to lie on April Fools' Day" or "That would be a good idea to say on April Fool's Day not today". Both of these sentences express your opinion on lying.
うそはエイプリルフールでない限りつかない方がいいと伝えたいなら、次のように言えます。
"You're only allowed to lie on April Fools' Day"（うそをついていいのはエイプリルフールだけだよ）
"That would be a good idea to say on April Fool's Day not today".（それは今日じゃなくてエイプリルフールに言った方がいいよ）
どちらの文でも、あなたのうそについての考えを伝えることができます。
You're only allowed to pull pranks on April Fool's Day
You are only allowed to lie on April Fool's Day.
When you want to tell someone they are only allowed to tell lies (jokingly) on April Fool's Day, then you may express it in the following ways:
-You're only allowed to pull pranks on April Fool's Day
-You are only allowed to lie on April Fool's Day.
嘘をついて良いのはエイプリルフールだけだと（冗談っぽく）言いたいなら、次のように表せます。
-You're only allowed to pull pranks on April Fool's Day
（イタズラをして良いのはエイプリルフールだけです）
-You are only allowed to lie on April Fool's Day.
（嘘をついて良いのはエイプリルフールだけです）
You can only tell these kinds of jokes on April's Fool day!
April Fool's is the only time you should tell these jokes!
I wouldn't play pranks on anyone if it isn't April Fools.
In the first example, I changed the word "lies" to jokes. You can be very direct in saying that "April Fools is the only day for pranks". The word prank is another work for joke. I distinguished lies from jokes because they are different. People play a lot of jokes or pranks on their friends on this day. Telling a lie is being dishonest about something and not really telling the truth about it later.
最初の例では "lies" を "jokes" に変えました。
"April Fools is the only day for pranks"（イタズラできるのはエイプリルフールだけ）は単刀直入な言い方です。
"prank" は "joke" の別の言い方です。"lies" を "jokes" と区別したのはこれらが異なるものだからです。
エイプリルフールにはたくさんの人が友達に "jokes"（イタズラ）または "pranks"（同）をしかけます。
"telling a lie" とは嘘をつくことをいい、これは必ずしもその後真実を伝えません。
You can tell as many lies as you want - but only on April Fool's Day
April Fool's Day is the one day in the year we can tell lies without feeling guilty
In a jokey way you want to tell someone that they can only tell lies on April Fool's Day. Well, however you say it, it seems to be an attempt at controlling someone, or influencing how they behave - or indicating that you do not trust them totally to tell the truth. However, if you want to say that, there are some examples above. Using 'we' as in the second example, softens the accusation somewhat.
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