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.