Why is it LE feu d'artifice and not les feux d'artifice?

"Toute la journée, on jouait à prendre la Bastille aux soldats du Roi jusqu'au soir où on marchait jusqu'à la clairière du parc dans laquelle le feu d'artifice avait lieu à la tombée de la nuit."

Because in this case, it is a single display of fireworks (the one in the clearing in the park) and in French "feu d'artifice" is a countable noun unlike in English where "fireworks" is a plural noun when describing the event and not the device.

See also several previous answers to the question about using the singular or plural form of "feu d'artifice", for example:
- https://progress.lawlessfrench.com/questions/view/why-feu-d-artifice-i-see-feux-d-artifice-used-in-multiple-examples
- https://progress.lawlessfrench.com/questions/view/why-feu-d-artifice-i-see-feux-d-artifice-used-in-multiple-examples