Green is the reason for the season.

Every spring, McDonald’s rolls out its bright green milkshake to celebrate St Patrick’s Day. The seasonal item is one of the restaurant’s most iconic offerings, despite the fact that it’s not always available.

To celebrate the Shamrock Shake’s 50th anniversary, McDonald’s published a press release about the drink’s history.

“We’ve been serving the iconic, cult-favorite Shamrock Shake for 50 years and it’s become synonymous with McDonald’s ever since,” said Linda VanGosen, McDonald’s vice president of menu innovation.

MCDONALD'S SHAMROCK SHAKES TO RETURN, OREO MCFLURRY VERSION TO MAKE DEBUT FOR 50TH ANNIVERSARY

According to the press release, the signature green dessert was created by a Connecticut McDonald’s owner in 1967. The Shamrock Shake made its national debut in 1970 and McDonald’s says it was “an instant success.”

“Every year customers eagerly await the return of the Shamrock Shake, and over the past five decades, getting a sip of this green legend has become a seasonal tradition for many,” said McDonald’s archivist Mike Bullington. “The shake’s unique history and widespread passion for this menu item have qualified the Shamrock Shake as a beloved cultural icon. We feel lucky to have such dedicated Shamrock fanatics, and hope to continue the legacy of this legendary treat for many more years to come.”

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In 1974, profits from Shamrock Shake sales were reportedly used to open the first Ronald McDonald House in Philadelphia after the company teamed up with the Children’s Hospital of Philadelphia and the Philadelphia Eagles.

According to McDonald's, the "World's Largest Shamrock Shake" was made in 2010 and stood 24 feet tall. It was poured into the Chicago River after a Chicago area Ronald McDonald House received a $10,000 donation.

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The minty shake was sold nationwide for the first time in 2012, according to McDonald's (it appeared at participating locations in various regions before that).

The 2020 Shamrock Shake is available at McDonald’s nationwide (and also Canada and Ireland) starting on Feb. 19.