Every year at the start of March Shamrock Shake fans rejoice as this minty milkshake returns to McDonald’s, but not many people know, or remember why this St. Patrick’s Day treat was launched back in 1966.

According to the McDonald’s website the Shamrock Shake was developed in 1970. McDonald's. However, according to reports on Delish.com, Harold Rosen, who owned a McDonald's in Connecticut, claims he invented the popular holiday drink in 1966.

The McDonald’s website says, "It all began with a little girl, a football team, and a visionary doctor."

The Philadelphia Eagles tight end Fred Hill's daughter was diagnosed with leukemia and began treatment. The Hill family were camping out in waiting rooms in hospitals and saw other families doing the same. They realized that many families travelling long distances to visit their sick family members had nowhere to stay and could not afford hotel rooms. The Hill family did something about it.

Hill and his team decided to raise some funds. The team's general manager, Jim Murray, called a friend in McDonald's advertising and suggested the team for the next promotional push. It just happened to be St. Patrick's Day.

The Shamrock Shake raised enough money to buy a four-story house which was the first Ronald McDonald House Charity.

Imagine -- the Shamrock Shake now has 19,763 fans on Facebook and is second in popularity only to the McRib, and that sandwich had a "Simpsons" episode dedicated to it.

People are obsessed. Last week when the St. Patrick’s Day treat hit the streets of New York WTOP tracked down some of its fans.

One woman tucking into her shake said, “It's delicious like heaven, yeah ...The mint tastes real good, and cool.”

Another got excited exclaiming, “Oh yeah, it's time for Shamrock Shakes.”

Love them or hate them they’re certainly an institution of St. Patrick’s Day.

Here’s the 1983 advert, featuring some seriously questionable Irish dance moves:

Here’s another Shamrock Shake advert:

McDonald's Shamrock shakeGoogle Images