No rule for this one
No ending or word group predicts the article of Monat. Learn it as a pair and say it out loud: der Monat.
der Monat
It's der Monat. Monat is masculine, plural die Monate.
Der Kurs dauert drei Monate.
The course lasts three months.
No ending or word group predicts the article of Monat. Learn it as a pair and say it out loud: der Monat.