AUSTRALIA - It's not exactly a national holiday - yet - but "gravy day" is being well celebrated in Australia, thanks to a song and plenty of internet jokes.

What is gravy day? It began with a 1996 song called How to Make Gravy by popular Australian singer Paul Kelly.

Its lyrics tell a fictional tale of a man in jail writing to his family about longing to be with them at Christmas.

That letter is penned on 21 December - a date that, in real life, has been deemed increasingly worth celebrating.

And many Australians did on Friday, especially on social media.

"GravyDay", "Paul Kelly" and "the 21st of December" were trending terms on Twitter, with many posts playing off How to Make Gravy's narrative. Joe, the song's made-up protagonist, sings about his family's traditions, such as relatives "driving down from Queensland" and "flying in from the coast".

The song has even been likened to "an Australian Christmas carol". Instead of northern-hemisphere references to cold things, Joe anticipates a hot Christmas Day.

"They say it's gonna be a hundred degrees, even more maybe," the lyrics read, "but that won't stop the roast".

His emotional messages to family - such as "give my love to Angus and Frank and Dolly" - have become a particular focus of fun.

Its popularity has inspired media outlets to investigate the protagonist's possible back story and the key question in the song: "Who's going to make the gravy?"

And just what is the best gravy recipe? According to the lyrics, it is: "Just add flour, salt, a little red wine, and don't forget a dollop of tomato sauce, for sweetness and that extra tang."

Some said they would follow that advice, but others questioned the recipe.