winter sun; winter sunshine
Refers to the sun or sunshine during winter, often with a sense of weak, gentle warmth.
The winter sun is shining in through the window.
Winter sunlight was streaming in through the window.
below-freezing day; day with sub-zero temperature
Meteorological term for a day when the minimum temperature falls below 0°C. Often used in weather forecasts.
Tomorrow will be a below-freezing day.
Today became the first sub-zero day of the year.
I drank tea while basking in the winter sun on the porch.
Sub-zero days have continued since last week.
冬日 is a meteorological term for a day whose minimum temperature falls below 0°C, the cold-weather counterpart to the hot-weather term 真夏日.
Compound of 冬 (ふゆ, 'winter') and 日 (ひ, 'sun' or 'day'), with rendaku voicing of ひ to び.