Generally, we don't use katakana for January, it's 一月(いちがつ / ichigatsu).
ONE MONTH is written as the same in kanji 一月 but it's read as ひとつき / hitotsuki, they don't mean the same.
Katakana is used for the words from other languages, names for person, the place and the country from overseas except some.
I hope this helps you.