Compound nouns: the last word decides
Vormittag der
Always In 98.9% of 6,721 common compound nouns, the article comes from the last word.
The last part decides the article: der Mittag, so der Vormittag.
der Vormittag
It's der Vormittag. Vormittag is masculine, plural die Vormittage.
Am Vormittag arbeite ich.
In the morning I work.
Vormittag der
Always In 98.9% of 6,721 common compound nouns, the article comes from the last word.
The last part decides the article: der Mittag, so der Vormittag.