Amanda Seyfried Got Drunk with Her Grandmother at “That” Legendary 2007 “Mamma Mia! ”Wrap Party (Exclusive)

What happens at a Mamma Mia! wrap party doesn’t always stay at the wrap party.

The 2007 Mamma Mia! wrap party became legendary, thanks to leaked photos of the cast having a blast — and rightfully so. “That was 18 years ago, I can't even do the math,” Amanda Seyfried recalls to PEOPLE. “But that wrap party specifically, I was drunk. Everybody was.”

It wasn’t just Seyfried, now 39, having a good time at the party; it was a family affair. “My grandmother was at that wrap party and she was slightly drunk,” she says. “She and my mom were there visiting. They always came everywhere I went and she lived until she was 99.”

Related: Amanda Seyfried Explains Why Fans Will Have to Wait for Mamma Mia 3: 'Always Working on It'

While the cast certainly enjoyed themselves, Seyfried says the wrap parties were a celebration for everyone — not just the actors. “Usually it’s about crew members kind of hooking up,” the Mean Girls actress recalls. “That’s the funny thing about wrap parties.”

Despite the hilarious memories, Seyfried admits that her wrap party days are behind her. “Wrap parties are places that I don’t go,” she explains. “I do not venture into the darkness of a wrap party so often.”

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However, there’s one upcoming event that might lure her back. “I might stick around for the wrap party for The Housemaid,” Seyfried reveals about her new movie, which is set to release on Christmas Day. When asked why she might make an exception, she smiles and adds, “Because Syd [Sweeney] loves a party, and I think I would want to celebrate with her.”

These days, staying up late looks a lot different for Seyfried. “The last time I stayed up past my bedtime was New Year’s because my daughter and I, with my best friend and her son, went to The Avett Brothers concert,” the actress says about her daughter Nina, 7, whom she shares with her husband Thomas Sadoski, along with their son Thomas, 4. “They have a New Year’s Eve concert every year, and we were in North Carolina at the concert. So Nina and I stayed up till 1:30 a.m. with my best friend and her 10-year-old son. It was just ridiculous, we were really blurry-eyed.”

Related: Amanda Seyfried Reveals Her 7-Year-Old Daughter Calls Her ‘Sophie’ After Becoming ‘Obsessed’ with Mamma Mia!: ‘She Loves It’ (Exclusive)

Though currently away from her kids while filming her upcoming Peacock thriller series Long Bright River — premiering March 13 — Seyfried says she finds ways to stay close to them.

“I got sent a picture of my kids dressed up ... like the Beastie Boys,” the actress tells PEOPLE about her most recent laughing fit. “I couldn’t believe a photo could make me laugh so much. It was perfection.”

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