winter, night, and rainy weather as ideal reading times
A yojijukugo referring to the three spare times (三余) traditionally considered ideal for reading: winter (the year's spare time), night (the day's spare time), and rainy weather (the season's spare time). Used in literary or classical contexts.
読書三余という言葉は、冬・夜・雨の日こそ読書にふさわしいという意味だ。
The phrase 読書三余 means that winter, night, and rainy days are the best times for reading.