Passover Coke is Coca-Cola specially made for the Jewish holiday of Passover, as it is not kosher to eat corn during Passover if you're observing that tradition, and Coke in America has been made with High Fructose Corn Syrup since sugar prices rose in the '80s (and corn subsidies got so ridiculous); Passover Coke is made with real Cane Sugar, and is comparable to the excellent Mexican Coke in flavor.
Passover Coke appears in stores in areas with large Jewish populations between one and three weeks before Passover. One can identify the Passover Coke by its lid: it is usually yellow, often has Hebrew writing on it, but the most important identifying mark is the two letters "KP" on the lid: "Kosher for Passover".