adverb, noun which may take the genitive case particle 'no'
Describes something that happens or is done each month. Can be used adverbially or as a no-adjective.
月ごとの会議は第一月曜日に行われる。
The monthly meeting is held on the first Monday.
この雑誌は月ごとに発行される。
This magazine is published monthly.
毎月 is more common and neutral for 'every month', while 月ごと emphasizes the monthly interval or cycle.
月次 is a formal, often business term for 'monthly', used in reports or accounting.
Compound of 月 (tsuki, 'month') and 毎 (goto, 'every'). The kanji 毎 is often replaced by okurigana ごと for readability.