12.10. Why do the Japanese use numbers for months?

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 lunisolar calendar (kyureki) months based on the moon. (See 12.6. What Western dates correspond to what Japanese dates?) Thus the eighth month is "falling leaves" (nearer to September) and the sixth month is "no water" (nearer to July).

In the original lunisolar calendar, there was also an extra "leap month" which was used when the year needed to have thirteen rather than twelve months, to keep pace with the seasons. See 12.6. 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
Satsuki Ugetsu	早月, 皐月雨月	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).