Well, as happy as possible, remembering that we're all one day closer to being gruesomely consumed by a cephalopod monster from the depths of the Pacific Ocean.
But it's not all bad news. Don't forget to sing that old favourite while remembering that some have not even heard the corrupting, eternally evil message of Cthulhu, "Do They Know It's Cthulhu Time At All? (Feed The World To Cthulhu)".
Anyway, we can all shiver together in fear of our impending consumption, and try to shorten the excruciating pain awaiting us by playing games that do please the Tentacled One, like "Pin the Tail on the Lurking Fear", or simply dancing naked round a fire that is fuelled by the mutilated body of a goat.
Lastly, it's important, I feel, that unspeakable terror isn't just for Cthulhu Day. It's for the rest of our miserable, wretched, carbon-based lives.
Happy Cthulhu Day, people.