McDonald’s is celebrating a big birthday. Or at least the birthday of a big thing.

Grimace, the large purple creature that’s been part of McDonald’s promotions since the 1970s, is celebrating a birthday, though its exact age is unclear. To mark the occasion, McDonald’s is rolling a new birthday meal.

Starting June 12, you can grab a Grimace Birthday Meal, with a choice of Big Mac or 10-piece Chicken McNugget and fries, as well as a new limited-edition purple shake inspired by Grimace’s “iconic color and sweetness” (that’s vanilla ice cream with berry flavoring.)

McDonald’s is remaining mum on an important question, however.

“What exactly is Grimace? Perhaps we’ll never know,” was McDonald’s best answer to the question.

Instead, the restaurant is banking on nostalgia.

“Our fans have amazing childhood memories of their birthdays at McDonald’s…and Grimace’s Birthday is all about paying homage to the amazing, fun moments we all share,” Tariq Hassan, Chief Marketing and Customer Experience Officer at McDonald’s USA, said in a statement. “We’re excited to put a modern spin on these memories to continue to drive McDonald’s relevance with a new generation. Grimace is the perfect lovable icon to have McDonald’s meet our fans at the intersection of nostalgia and culture.”

The restaurant chain is also launching a Grimace-themed video game and Grimace merch available at GoldenArchesUnlimited.com.

In lieu of gifts, Grimace is asking fans to donate to Ronald McDonald House Charities. On June 13 and 14, fans can visit McDonald’s Instagram stories and share a picture of their favorite birthday memory via the “add yours” sticker. McDonald’s will donate $5 to RMHC up to $200,000 for every picture shared.