The story
The proper noun comes from an ellipsis of the fourth day of July. The common noun is derived from the proper noun.
Etymology from Wiktionary, CC BY-SA 4.0.
Fourth of July
Fourth of Ju·lyToday it means: A cocktail containing one part grenadine syrup, one part vodka, and one part blue curaçao, reminiscent of the red, white, and blue colors of the United States flag, often poured in layers so that the colors do not mix when the drink is served. Full definition →