beginning of winter
Obsolete term for the start of winter, used in classical Chinese-influenced contexts. In modern Japanese, 初冬 (しょとう) is the common word.
tenth month of the lunar calendar
Refers specifically to the tenth month in the traditional lunar calendar, roughly corresponding to November. This usage is obsolete outside historical or literary contexts.
In the old calendar, 孟冬 referred to the time around the beginning of winter.
孟冬 was used as another name for the tenth month of the lunar calendar.
From Chinese 孟冬 (mèngdōng), where 孟 means 'first' or 'beginning' and 冬 means 'winter'. The term was adopted into classical Japanese and is now obsolete.