Translation:Song and dance
In a word, countability.
In English, putting an article like "a/an" or "the" with a noun implies there is only one of the noun. Omitting the article on a singular noun, like "song" or "dance", makes it uncountable; it transforms the noun from a discrete object or objects of which one can count individuals (one song, two songs, etc.) into a collective stuff you can have a little or a lot of (like water, or sugar, or molasses; you can't usually drink "a water", but you can drink some or a lot of water).
Thus, "song and dance" refers more generally to singing and dancing than to a specific song or a specific dance; "there was song and dance before the fireworks" means some people did some singing and some dancing before the fireworks, whereas "there was a song and a dance before the fireworks" means one song was sung and one dance was danced before fireworks happened.