Midsummer can be broken up into the following:
Mid - this means middle
Summer - which is the warmest season of the year
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Examples
It was a hot midsummer's day.
The misdsummer heat is so unbearable.
Midsummerはいくつかの意味に分けられます。
Mid - 真ん中の、途中の
Summer - １年で一番あたたかい季節
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例
It was a hot midsummer's day. 真夏の暑い日です。
The misdsummer heat is so unbearable.真夏の暑さは耐えられないほどです
"dog days of summer" is an expression used in talking about the very hot days in the middle of summer.
"mid-" is a shortened version for "middle" that can be used for days, weeks, months, or seasons.
"dog days of summer" (盛夏）とは真夏の暑い日を表現する時に使います。
"mid-"とは"middle"(真ん中）の省略で、日、週、 月や季節に使われます。
When you want to talk about the middle of summer (when it's usually very hot), then you can say:
-It is the height of summer and the sun is so hot.
-I don't like mid-summer because it gets quite hot and humid.
「真夏（普通一番暑いとき）」について話すときの表現です：
-It is the height of summer and the sun is so hot.
真夏で、太陽がすごくあつい。
-I don't like mid-summer because it gets quite hot and humid.
すごく蒸し暑くなるので真夏は好きではない。
The 'height of' something is described as the peak period of something. It has nothing to do with how tall something is. For example:
Prices were expensive, it was the height of the market
It was the height of the party, it was a lot of fun
「height」は「ピークの時期」を表します。物の高さとは関係もありません。
例：
Prices were expensive, it was the height of the market
〔訳〕価格は高く、市場の最盛期だった
It was the height of the party, it was a lot of fun
〔訳〕パーティーの盛り上がりは最高潮で、とても楽しかった
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