No rule for this one
No ending or word group predicts the article of Tag. Learn it as a pair and say it out loud: der Tag.
der Tag
It's der Tag. Tag is masculine, plural die Tage.
Ich arbeite acht Stunden am Tag.
I work eight hours a day.
No ending or word group predicts the article of Tag. Learn it as a pair and say it out loud: der Tag.