No rule for this one
No ending or word group predicts the article of Abend. Learn it as a pair and say it out loud: der Abend.
der Abend
It's der Abend. Abend is masculine, plural die Abende.
Am Abend lerne ich Deutsch.
In the evening I study German.
Am Abend rufe ich meine Eltern an.
In the evening I call my parents.
No ending or word group predicts the article of Abend. Learn it as a pair and say it out loud: der Abend.