The simple answer to why we celebrate a Cocktail Day is – somebody got curious about the definition of cocktail, an editor responded to their query on 13th May 1806, and the day has ever since been set aside for Cocktails.

On this day, more than 200 years back, the editor of ‘The Balance and Columbian Repository’ responded the first-known definition to a cocktail:
“..cock-tail, then, is a stimulating liquor, composed of spirits of any kind, sugar, water, and bitters – it is vulgarly called a bittered sling, and is supposed to be an excellent electioneering potion, in as much as it renders the heart flout and bold, at the same time that it fuddles the head. It is said also, to be of great use to a democratic candidate: because, a person having swallowed a glass of it, is ready to swallow anything else.”
Phew.
I would’ve probably sounded something like this if I tried explaining what a cocktail is – “Mixed Drink. Has Alcohol. Very good taste.”
While we might not be able to put together words to explain the brilliance of cocktails, an entire day has been set aside to celebrate the art of cocktails. So go fancy, mix up a cocktail with your friends, family, neighbour, paper-man, or just about anyone around for a drink.