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.

Japanese has names for the months, but these have fallen into disuse. The month names, when used with the old-style Japanese calendar, don't correspond to the modern months, which are equivalent to January, February, etc., but to the old-style lunar calendar months. Thus the eighth month is "falling leaves" (nearer to September) and the sixth month is "no water" (nearer to July).

Old name in romanized Japanese	Old name in kanji	Old name meaning	New name in romanized Japanese	New name in 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
Satsuki	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. It is also called kamiarizuki (神在月), "month of having gods", in the area of the Izumo Grand Shrine, Izumo Taisha (出雲大社) in Shimane 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

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).

Acknowledgements

Edited from a newsgroup post by NAKANO Yasuaki. Esben Lundsgaard corrected an error in the location of Izumo Shrine.

External links

Alternative month names, from the Kanji Kentei website, contains many more names for the months. (in Japanese)

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