January
first month of the lunar calendar
「一月」 has another reading with different meanings.
one month
今年の一月はとても寒い。
January this year is very cold.
旧暦の一月は今年二月中旬から始まる。
The lunar first month begins in mid-February this year.
一月の終わりに旅行を計画した。
I planned a trip for the end of January.