When talking about classic slasher icons, it's hard not to mention the likes of Michael Myers' terrifying figure or Jason Voorhees' iconic "ch-ch-ch-ah" that plays whenever he's on screen. However, the latter's franchise has shown that consistency isn't a prerequisite when it comes to the hockey mask-wearing menace that is Jason, who's been everywhere from Manhattan to space, all to kill in the name of his mother.

With each Friday the 13th film, the storyline consistently became watered down and focused on Jason's next kill over maintaining the core theme established in the original movie. The significance of its aforementioned date served as the catalyst for one of the franchise's biggest continuity issues.

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The original Friday the 13th took place in the summer of 1979. Though an official date hasn't been set, fans have theorized that it should have taken place in April or July, as those were the only two months in the year when the 13th fell on a Friday. The film also stressed the importance of the date as it became the main motivation for Pamela's killing spree. In Friday the 13th, she tells the teens, "You see, Jason was my son, and today is his birthday." In a standalone film, this would work perfectly, but as the franchise continued, Jason's lore grew, and even his birthdate contradicted the supposed date of the original film.

As the franchise's universe expanded with the release of Friday the 13th: The Final Chapter, it created further inconsistencies. One new revelation was that Jason was born on June 13, 1946 -- a Thursday. This blatantly contradicted the birthdate that was theorized by fans. The new date has the first film occurring on Wednesday, June 13, 1979, even though July 13 was on a Friday and served as the simplest conclusion for fans to decide on, considering the film's title.

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The alterations continued following the appearance of Pamela Voorhees' resting place. A shot in The Final Chapter revealed Pamela's headstone stating that she died in 1980, even though Friday the 13th was set in '79. This decision was likely due to the film being theatrically released in 1980. The Final Chapter created a continuity error that still requires clarification as there are no definitive answers on when the film's events took place.

Whether trying to capitalize on themed slasher films like John Carpenter's Halloween or simply trying to add an air of mystery, it has been proven that director Sean S. Cunningham's Friday the 13th inadvertently created confusion that is still discussed to this day. Whether it was due to a rushed production on sequels that ruined consistency or an overall lack of attention to detail, it is clear that the main focus was on how to bring Jason back from the dead rather than why the series was called Friday the 13th in the first place.

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