noun
hot summer day
General term for a day that feels hot in summer, without a strict temperature definition.
今日は夏日で、外に出ると汗が出る。
Today is a hot summer day; stepping outside makes you sweat.
noun
day with max temp ≥ 25°C
Meteorological definition: a day when the highest temperature reaches 25°C or above. Compare with 真夏日 (≥30°C) and 猛暑日 (≥35°C).
今年は夏日が例年より早く、4月に25度を超えた。
This year, the first day with a high of 25°C came earlier than usual, exceeding 25°C in April.