In case you haven’t been on the internet for a while, Oct. 3rd is Mean Girls Day — the official holiday celebrated by fans of the teen comedy.

“Mean Girls” debuted more than 15 years ago. So, how is it still so celebrated among millennials and teenagers today?

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For starters, it’s funny, irreverent and very quotable — so much so that it’s difficult to avoid cultural references to it — even if you haven’t seen it before.

But beyond just humor, the film’s portrayals of high school and teenage girls is rooted in anthropology. I mean, it’s not subtle — Lindsey Lohan’s character literally compares the dynamics of teenagers in high school to animals in the wild.

Because of that framing device, the movie’s imitations of high school popularity, “burn book” rumors and cafeteria cliques are absurd — but they’re also relatable and informative.

There’s a reason for that. “Mean Girls” was actually based on the nonfiction book, “Queen Bees and Wannabes.” The author, Rosalind Wiseman, is a renowned speaker on parenting and anti-bullying.

In a 2014 interview with The Atlantic, Wiseman said the film “gave archetypes of people’s behavior, and it sort of started off as stereotypes, but it got into the bigger and more complicated and more nuanced dynamics between all of the characters.”

Today, the portrayal of these dynamics still hold relevance. No longer just a movie, “Mean Girls” has spawned a Tony-nominated Broadway musical and countless rumors of direct spin-offs and sequels.