「定休日」は英語で、regular day off
regular holidayと言いますが、
「～（曜日に）[閉まっている](https://eikaiwa.dmm.com/uknow/questions/73036/)」のように
言う方が一般的です。
例文
This cafe is normally closed on Mondays.
この喫茶店は月曜日が定休日です
We are regularly closed on Mondays.
当店は月曜日が定休日です。
定休日をnormally closed 、
regularly closedで表現しています。
regular day off の regular は「[定期的な](https://eikaiwa.dmm.com/uknow/questions/46626/)」「通常の」などのニュアンスがあります。
day off は「休日」と言えます。
参考になれば幸いです。
The three sentences you see above are excellent ways to describe a day of the week a store is closed. In the first sentence you will see the adverb normally and in the third sentence you will see the adverb always. Although there are many definitions for these two words, in these sentences both mean at all times. These are words that are common in our everyday conversation and would make a great addition to your vocabulary.
三例とも、[お店](https://eikaiwa.dmm.com/uknow/questions/5391/)が休業する曜日を表すすごくいい言い方です。
一つ目の例文には「normally（副詞）」、三つ目の例文には「always（副詞）」が使われています。この二つにはいろいろな意味がありますが、例文ではどちらも「[いつも](https://eikaiwa.dmm.com/uknow/questions/32890/)（at all times）」という意味です。日常会話でよく使われる単語ですので、是非覚えてください。
There are a number of different ways that one may refer to a shop being closed on a particular day.
You could simply say, "They are closed today." or "They are not open."
Many shops will display signs that list their opening hours. Days off will read similarly to "SUN: CLOSED".
The people working for the business are more likely to refer to it as a "day off", as in a day that they are off work, or a "non-working day". "Sorry, we can't receive the delivery on Monday as its our day off."
If referring to before or after the time when the shop is open in the current day, you could use "out of office hours". An example sentence: "We won't be able to do it after 7pm as it's out-of-office hours."
休みのお店を表す言い方はいろいろあります。
シンプルに：
They are closed today.
（今日は休みです）
They are not open.
（休みです）
と言うことができます。
お店には営業時間を示した看板が置いてあることが多いです。定休日は：
"SUN: CLOSED"
（日曜日休み）
のように書かれます。
そこで働いている人は「day off（休日）」や「non-working day（休日）」と言うことが多いと思います。
〔例〕
"Sorry, we can't receive the delivery on Monday as it's our day off."
（ごめんなさい、月曜日は休みなので配送は受けられません）
店が開店する前または閉店した後の時間について言う場合は「out of office hours」が使えます。例えば：
"We won't be able to do it after 7pm as it's out-of-office hours."
（営業時間外なので、午後7時以降はできません）
If the shop closes every Wednesday or any other day of the week regularly, that day can be called a 'non-business day'. This means that on such a day, the shop does not carry out any business because it is closed. The shop can also put up notices on boards stating that it is 'closed on Wednesdays' or any other day.
You may tell someone as follows:
Every Wednesday is a non-business day for us.
or
The shop is closed on Wednesdays
or
We are closed every Wednesday.
店に水曜日など定期的に休む曜日があるなら、その日は「non-business day」と言えます。これは、その日は店が閉まっていて、営業していないことを表します。
店に「Closed on Wednesdays（水曜日休み）」などと書かれた案内が置かれることもあります。
例：
Every Wednesday is a non-business day for us.
（毎週水曜日は定休日です）
The shop is closed on Wednesdays
（その店は水曜日休みです）
We are closed every Wednesday.
（毎週水曜日は定休日です）
Closed on Saturdays shows that every Saturday the place/store/restaurant is closed.
Ex:
Caller: (calls a restaurant) Hello I would like to make reservations for next Friday.
Employee: Sorry sir, We are closed on Fridays, how about Saturday or Thursday?
Caller: Saturday evening will work fine thank you,
Closed on Saturdays' は、お店やレストランが「毎週土曜日」に休みであることを表します。
例：
電話をかけてきた人:（レストランに電話して）Hello I would like to make reservations for next Friday.（こんにちは、金曜日に予約をしたいです）
店員: Sorry sir, We are closed on Fridays, how about Saturday or Thursday?（すみません、当店は金曜日は定休日です。土曜日か木曜日ではどうですか）
電話をかけてきた人: Saturday evening will work fine thank you.（土曜の夜は都合がいいです、ありがとうございます）
The days when a store is not opened, those are called non-trading days or a closed (day).
For example, you can say:
-Sunday is a non-trading day for Walmart in our town
-The shop is closed on Sundays
お店が開いていない日は、「non-trading days」または「a closed (day)」と言います。
例えば、以下のように言えます：
Sunday is a non-trading day for Walmart in our town．
（日曜日は、私の町のウォールマートの定休日です。）
The shop is closed on Sundays.
（その店は、毎週日曜日定休日です。）
Most commonly you would just state "The store is closed on Sundays" or the "The restaurant is closed every Monday and Tuesday."
These are also known as "non-trading" or "non-business" days.
最も一般的な言い方としてはシンプルに：
"The store is closed on Sundays."
「その店は日曜日は休みです」
"The restaurant is closed every Monday and Tuesday."
「そのレストランは毎週月曜日と火曜日は休みです」
などと言います。
これらは「non-trading day（休業日）」「non-business day（休業日）」としても知られます。
When something is closed it is not open
for example a shop could be closed on Mondays to give the owners or staff a break or they could be closed for a week if they are going on holiday
A shop can sometimes have a regular day off in a week that it closes for example in Malta a lot of shops are closed or not open on a Sunday as this is a family day so they stay shut and spend the day together relaxing instead of working
日本 
アメリカ合衆国 
オーストラリア 
南アフリカ 
イギリス 
