Learn German: Alle großen Verlage kommen auf die Buchmesse. - All big publishers are coming to the book fair.

After the word alle (all), adjectives in the plural take the weak ending -en. This is because alle acts like a definite article (like the plural die), so it already shows the case and gender, meaning the adjective doesn't have to.

Would the adjective ending change if I used viele (many) instead of alle?

Yes! Words like viele, einige, and mehrere act like regular adjectives, not definite articles. If you said 'many big publishers', both words would take the strong plural ending -e, making it Viele große Verlage.

Why is it auf die Buchmesse (accusative) and not auf der Buchmesse (dative)?

The preposition auf is a two-way preposition. Because the verb kommen shows movement toward a destination (answering 'Where to?'), we use the accusative case (die). If the publishers were already at the fair setting up a booth (answering 'Where?'), you would use the dative case (auf der Buchmesse).

Why do we use the preposition auf for a book fair instead of zu?

In German, you typically use auf for public events, markets, fairs, and parties. It is as if you are going 'onto' the event grounds. Saying zur Buchmesse (to the book fair) is also perfectly fine, but auf is very natural and common for this type of gathering.