a/an is omitted
A Before plural nouns.
a/an has no plural form. So the plural of a dog is dogs, and of an egg is eggs.
B Before uncountable nouns
C Before names of meals, except when these are preceded by an adjective:
We have breakfast at right.
He gave us a good breakfast.
The article is also used when it is a special meal given to celebrate something or in someone’s honour:
I was invited to dinner (at their house, in the ordinary way) but
I was invited to a dinner given to welcome the new ambassador.