National Joe Day is celebrated on March 27th of each year. The absence of any clear origin for the use of joe to mean coffee has, as usual, led to stories being created to explain where it came from.
A persistent one alleges that it derives from the ban imposed by Josephus “Joe” Daniels, Secretary of the Navy during World War I, on serving alcohol aboard US Navy ships, except on very special occasions. Coffee, it is said, became the beverage of choice and started to be called Joe in reference to him. The problem with this story is the dates. Cup of joe first appears in print in 1930 but the order to ban alcohol — General Order 99 — was issued on 1 June 1914. It banned officers’ wine messes, which had only been permitted since 1893; ships had otherwise been dry since the spirit ration was abolished in 1862. It seems hardly likely that the loss of a wine mess limited to officers on board otherwise alcohol-free ships would have led to a nickname for coffee that only started to be written down 16 years after the order.
Professor Jonathan Lighter, in the Random House Historical Dictionary of American Slang, leans towards another story: that it came from the Stephen Foster song Old Black Joe, with the resultant mental link between black and coffee. It is true that the song — written in 1860 — was extremely popular at one time, but it makes no reference to coffee, so linking the two is implausible.
The most boring, but most probable, suggestion is that it is a modification of java or jamoke for coffee, perhaps under the influence of one or other of the many expressions at the time that contained the word Joe — for example, “an ordinary Joe” (though “GI Joe” for an enlisted man in the US military is from the next decade). It is significant that an early example appears in 1931 in the Reserve Officer’s Manual by a man named Erdman: “Jamoke, Java, Joe. Coffee. Derived from the words Java and Mocha, where originally the best coffee came from”.