singularity day; statistically anomalous weather day
A technical term in climatology for a calendar day on which a specific weather phenomenon (e.g., heavy rain, clear skies) occurs with a statistically significant frequency compared to surrounding days. Often used in Japanese weather folklore and meteorological statistics.
November 3rd is known as a singularity day for clear weather.
According to the Japan Meteorological Agency's data, this day is a singularity day for rain.
Compound of 特異 (tokui, 'singular, peculiar') and 日 (bi, 'day'). The term is used in Japanese meteorology to describe days with statistically unusual weather patterns.