noun
day with max temperature below 0°C; cold winter day
Meteorological term for a day when the highest temperature stays below freezing. Often used in weather forecasts and winter descriptions.
今日は真冬日で、最高気温が氷点下のままだ。
Today is a day with the temperature staying below freezing, and the high remains below zero.
真冬日が続くと、水道管が凍る心配がある
When cold winter days continue, there is a risk of water pipes freezing.
真夏日 is a midsummer day with a maximum temperature of 30°C or above, contrasting with 真冬日's freezing conditions.
Compound of 真 (ま, 'true, pure') + 冬日 (ふゆび, 'winter day'), literally 'true winter day'. The prefix 真 intensifies the meaning to indicate a day that is fully wintry, i.e., one where the temperature remains below freezing.