“To have lunch” is correct when we are speaking about having lunch in general. “A lunch” is only used with compound nouns that include the word lunch.
“Lunch” with no article is the most common form of having a meal in the middle of the day. The word “lunch” is commonly colocated with the verbs “to have”, “to eat” or “go(out) for”.
Let’s have lunch on the balcony.
Let’s eat lunch now.
Why don’t we go out for lunch today?
“A lunch” is possible in English. It usually acts as an adjective(in the form of a compound noun) in English and describes the noun that follows.
Don’t forget to bring a lunch box.
I need to take a lunch break.
In this case, we use the article “a” because we are referring to “one of many”
In the first example above, there might be many lunch boxes so you need to choose one.
This could work in many examples. Think about “ a lunch meeting”. There could be many types of meetings and “a lunch meeting” is just one of them.
We do not use “a lunch” alone(unless “meeting” is implied)
I want to eat a lunch.
I have a lunch with the CEO later. (meeting is implied)
We use “the lunch” when referring to a specific lunch. We are talking about the food for “the lunch”
We often use “the lunch” when we want to highlight the food that we ate during our lunch.
I enjoyed our stay at the hotel. The facilities weren’t great but the lunch was fantastic.
We also use “the lunch” when referring to compound nouns when both people know what we are talking about.
Do you bring the laptop for the lunch meeting?
Have a nice Lunch(a +adjective +Lunch)
When you add an adjective before lunch, you also need to a or an.
Have a great lunch.
I had an amazing lunch, Thank you.
What an incredible lunch!
Lunch or Lunches(Plural)
The plural of “lunch” is either lunch or lunches.
We use lunch as the plural when we are talking about lunch in general if we all eat the same lunch.
What are we having for lunch?
We use “lunches” when we are talking about different types of lunches. This could happen if a group of people eat different lunches or think about different “lunches” for different times.
I am so prepared! I have made all the lunches for the week.
Did the children enjoy their packed lunches? (Their parents prepared their lunches at home)
Luncheon is a formal word that also means Lunch but is extremely rare nowadays. The only time you might see the word “luncheon” is when we talk about “luncheon meat” which is canned meat.