“Green River” is about to take on a whole new meaning this weekend as the Chicago River once again gets dyed green for St. Patrick’s Day.

The iconic event this year takes place Saturday, March 16. The Chicago Plumbers Union, Chicago Journeymen Plumbers Local 130, has been dyeing the Chicago River green in honor of the city’s St. Patrick’s Day celebrations for decades, with this year set to mark the 69th.

Here’s how they do it (although they don’t reveal all their secrets), along with how to watch and more.

When is the Chicago River dyeing?

According to officials, the river dyeing this year will take place at 10 a.m. on March 16, just one day before the St. Patrick’s Day holiday.

Watch the Chicago River dyeing live

While there will be plenty of viewing spots for those who in person, the event is often highly attended, which means plenty of crowds to navigate.

For those looking to go in person, officials do remind the public that the lower level of the riverwalk will be closed on that date, with no access available via public stairwells in the area. Instead, revelers are asked to watch the festivities on Upper Wacker Drive, with the best views available between Columbus and Fairbanks.

The St. Patrick’s Day Parade will begin at approximately 12:15 p.m. on March 16, with route information to come. Typically, the route moves along Columbus Drive from Balbo to Monroe Street, allowing access to numerous downtown attractions before and after the parade.

Why is Chicago’s River dyed green?

The tradition, adhered to for decades, will coincide with the city’s annual St. Patrick’s Day parade.

The story goes, in 1961, a man by the name of Stephen Bailey — the business manager of the plumbers union — was approached by “one of his plumbers who was wearing some white coveralls,” a post by Local 130 says. It was then the union says, that Bailey noticed the overalls had been stained or dyed with “a perfect shade of green,” or “an Irish green to better describe it.”

When wondering how the coveralls could have turned such a tint, the tale continues, Bailey and his plumbers discovered that it was from to the dye used to detect leaks in the river.

“That’s when Mr. Bailey bellowed,” the Union declares, “Call the mayor … we will dye the Chicago River green!”

And there you have it.

Who dyes the River green?

For those who have never seen the process in person, each year the Chicago Plumbers union embarks on boats that putter along the Chicago River, with a rather curious concoction that sprays dye out of plumping pipes and spout pumps.

But the mysterious mixture doesn’t come out green.

“If you were watching this for the first time you would think this is a mistake or a bad joke,” the union said. “You see the dye is orange, and its initial color on the surface of the river is orange, and you would think to yourself what ‘heathen would do something like this.'”

However, once the dye sets in, the color “in a stroke of luck” turns, and the “true color magically appears,” the union said.

What’s in the dye?

The actual recipe has never quite been revealed, the plumbers’ union won’t reveal its secrets.

The plumbers union, Choose Chicago says, “still holds the river-dyeing honors today.” But you won’t be able to find their recipe anywhere.

There is one thing the city does say about the dye, however.

“Their environmentally friendly dye formula remains a closely kept secret,” Choose Chicago says.