Why do the Japanese use numbers for months?

The months in Japanese are usually referred to using numbers, Ichigatsu (一月), Nigatsu (二月), Sangatsu (三月), and so on. Japanese also has names for the months, but these have fallen into disuse. The month names don't correspond to the modern months, which are equivalent to January, February, etc., but to the months of the old-style lunisolar (moon-based) calendar which is now called the kyūreki (旧暦), "old calendar". (See also What Western dates correspond to what Japanese dates?) Because the lunisolar New Year, similar to the "Chinese New Year", is later in the year than the Western-style New Year currently used, the eighth month is "falling leaves" (nearer to September) and the sixth month is "no water" (nearer to July).

An extra "leap month" in the lunisolar calendar was used when the year needed to have thirteen rather than twelve months, to keep pace with the seasons. See What Western dates correspond to what Japanese dates?

Old name (romanized) Old name (kanji) Meaning of old name New name (romanized) New name (kanji)
Mutsuki 睦月 Month of harmony Ichigatsu 一月
Kisaragi 如月 Month of wearing extra layers of clothes Nigatsu ニ月
Yayoi 弥生 Month of growth Sangatsu 三月
Uzuki 卯月 Month in which unohana (卯の花) blooms Shigatsu 四月
早月, 皐月
Month of planting rice sprouts Gogatsu 五月
Minazuki 水無月 Month of no water Rokugatsu 六月
Fumizuki, Fuzuki 文月 Month of letters Shichigatsu 七月
Hazuki 葉月 Month of leaves Hachigatsu 八月
Nagatsuki 長月 Autumn long month Kugatsu 九月
Kannazuki, Kaminazuki 神無月 Month of no gods. Called kamiarizuki (神在月) in Izumo (Tottori prefecture) because the gods were said to gather there. Jūgatsu 十月
Shimotsuki 霜月 Month of frost Jūichigatsu 十一月
Shiwasu 師走 Month of busy priests Jūnigatsu 十二月
Uruuzuki, Jungetsu 閏月 Leap month

From NAKANO Yasuaki:

It has been very popular in Japan to name the months by numbers. Months in the old calendar had their own names, such as Mutsuki, Kisaragi, etc., but these names appeared only in novels, poems or literary expressions. I think the official documents called months by numbers (in very ancient ages they might have been named using the Chinese zoological sequence).

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