Arizona Diamondbacks v. New York Mets
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Both New York baseball clubs will wear commemorative hats to honor first responders for their respective games on Friday, the 19th anniversary of the Sept. 11 terrorist attacks. MLB had not previously allowed either the Mets or the Yankees to wear FDNY and NYPD caps on the field since that Sept. 21, 2001 game at Shea Stadium.

The Mets are set to face the Blue Jays in Buffalo Friday night, while the Yankees will begin a four-game series against the Orioles at Yankee Stadium. Here's a look at the Yankees cap honoring the New York City Fire Department:

The push to get MLB's approval on a commemorative in-game act like this was led by Mets first baseman Pete Alonso. Last season, the 2019 Rookie of the Year organized and purchased the Mets special commemorative 9/11-themed cleats that they wore in-game without the permission of the league office.

The cleats depicted images of the American flag, plus the Sept. 11, 2001 date, "We will never forget" and NYPD/FDNY/EMT lettering. Alonso later donated his custom cleats to the National September 11 Memorial and Museum in downtown Manhattan. On Sept. 11 last year, the Mets were allowed to wear first-responder hats during batting practice, but had to switch to their team hats for the game.

"We're really excited that this year we're gonna be able to wear commemorative hats in the game," Alonso told WFAN ahead of Friday's game. More from Alonso:

So years prior, we weren't allowed to. I think because of last year with the cleats and also [Mets owner] Jeff Wilpon was in the commissioner's ear about letting us wear the hats to represent people who responded that day -- FDNY, NYPD, Sanitation, Port Authority, and EMTs.

Without those people, who knows what could've happened. So we're gonna be commemorating those people with five different hats. You're gonna have people representing those different organizations on the field today. I'm really excited that we get to represent everybody that was there to help and save others.

Both teams helped the country heal following the attacks.

On Sept. 21, 2001, the Mets returned to host the Braves at Shea Stadium for the first professional sporting event following the attacks. Hall of Famer and former Mets catcher Mike Piazza hit a go-ahead home run in the emotional game just ten days after 9/11.

"Many of you give me praise for the two-run home run on the first game to push us ahead of the rival Braves, but the true praise belongs to police, firefighters and first-responders who knew that they were going to die but went forward anyway," Piazza said during his Hall of Fame induction speech in 2016.

The Yankees, meanwhile, reached the 2001 World Series against the Arizona Diamondbacks. Prior to Game 3, President George W. Bush threw out the ceremonial first pitch at Yankee Stadium.

Next season, the Mets and Yankees will play each other at Citi Field on the 20-year anniversary of 9/11. The two teams have never played on Sept. 11 and only once before -- in 2014 -- have both clubs been at home in New York on that date.