McDonald's has introduced and quietly killed many dishes over the years (remember McDonald's pizza?), but there's a core group of items that have held their spot on the menu for decades. Listed alongside the Big Mac and McNuggets is the Filet-O-Fish—a McDonald's staple you may have forgotten about if you're not the type of person who orders seafood from fast food restaurants. But the classic sandwich, consisting of a fried fish filet, tartar sauce, and American cheese on a bun, didn't get on the menu by mistake—and thanks to its popularity around Lent, it's likely to stick around.

According to Taste of Home, the inception of the Filet-O-Fish can be traced back to a McDonald's franchise that opened near Cincinnati, Ohio in 1959. Back then the restaurant offered beef burgers as its only main dish, and for most of the year, diners couldn't get enough of them. Things changed during Lent: Many Catholics abstain from eating meat and poultry on Fridays during the holy season as a form of fasting, and in the early 1960s, Cincinnati was more than 85 percent Catholic. Fridays are supposed to be one of the busiest days of the week for restaurants, but sales at the Ohio McDonald's took a nosedive every Friday leading up to Easter.

Franchise owner Lou Groen went to McDonald's founder Ray Kroc with the plan of adding a meat alternative to the menu to lure back Catholic customers. He proposed a fried halibut sandwich with tartar sauce (though meat is off-limits for Catholics on Fridays during Lent, seafood doesn't count as meat). Kroc didn't love the idea, citing his fears of stores smelling like fish, and suggested a "Hula Burger" made from a pineapple slice with cheese instead. To decide which item would earn a permanent place on the menu, they put the two sandwiches head to head at Groen's McDonald's one Friday during Lent.

The restaurant sold 350 Filet-O-Fish sandwiches that day—clearly beating the Hula Burger (though exactly how many pineapple burgers sold, Kroc wouldn't say). The basic recipe has received a few tweaks, switching from halibut to the cheaper cod and from cod to the more sustainable Alaskan pollock, but the Filet-O-Fish has remained part of the McDonald's lineup in some form ever since. Today 300 million of the sandwiches are sold annually, and about a quarter of those sales are made during Lent.

Other seafood products McDonald's has introduced haven't had the same staying power as the Filet-O-Fish. In 2013, the chain rolled out Fish McBites, a chickenless take on McNuggets, only to pull them from menus that same year.

[h/t Taste of Home]