The 'lunch' group
These nouns refer to meals. With all these nouns except meal, the noun group does not usually begin with a determiner.
The atmosphere at lunch was relaxed.
The hospitals provide a choice of healthy dishes at every meal.
Lunch and dinner are sometimes used with a determiner when they refer to a formal social event.
at breakfast at dinner at lunch at supper at tea
At a dinner held in his honour by President Vo Chi Cong, Mr Suharto said the settlement would open a new chapter in the history of South East Asia.
Meal is used as a normal count noun.