McDonald's bright-green Shamrock Shake, a minty beverage only available around St. Patrick's Day each year, has a cult following that rivals that of "The Rocky Horror Picture Show" and "short-lived TV series "Firefly" (though possibly not McDonald's other fan-favorite seasonal menu item, the McRib). The shake counts over 21,000 fans on its unofficial Facebook page and famous fans like Jimmy Fallon have paid public tribute to its allure.¬†The shake's availability varies from franchise to franchise, so Shamrock cultists have crowdsourced a Shamrock Shake locator on the web.

What's all the fuss about? This vintage Shamrock Shake commercial from 1983 may hold the answer:

Free leprechaun hats with every shake? So that's their secret.