NEW YORK -- A year after Pete Alonso sparked renewed interest in the Mets’ desire to wear first responder caps on Sept. 11, Major League Baseball granted permission for both the Mets and Yankees to wear the caps on the 19th anniversary of the World Trade Center attacks.

Mets and Yankees players took the field wearing caps from the NYPD, the FDNY, the Port Authority Police Department, the Office of Emergency Management and the Department of Sanitation -- not just during batting practice, as had become customary in recent years, but during their games against the Orioles and Blue Jays, respectively.

“I love this city,” Alonso said. “I love everything that New York has to offer. … This is not just a dark spot for New York City, but for the entire country as a whole. This is probably one of the worst days in our country’s history. Even today, we’re still being impacted by it.

“I’m not saying I completely understand it, because I didn’t live through that day in New York. But for me, I think that seeing how this thing has affected fellow New Yorkers, honestly, it’s incredible in the worst way. I just want to represent those and just recognize everybody that contributed to help save lives and impact and make incredibly selfless decisions.”

The Mets famously wore the caps for the first time when they faced the Braves on Sept. 21, 2001, after several of their players traded Mets hats for first responder caps during visits to Ground Zero. They wore them sporadically during games over the next six seasons, but did not do so after 2007.

Last year, Alonso expressed interest in again donning the caps in a game. When he was told the Mets would wear them only during batting practice, he instead designed and purchased commemorative Sept. 11 cleats for himself and each of his teammates. Alonso subsequently donated those spikes to the National September 11 Memorial & Museum, all while continuing to express interest in wearing first responder caps during future Sept. 11 games.

Mets chief operating officer Jeff Wilpon took up the cause, spearheading conversations with MLB Commissioner Rob Manfred to make it happen.

The effort was personal to Alonso, who has taken significant interest in Sept. 11 initiatives since moving to New York. Last summer, Alonso donated 5 percent of his $1 million Home Run Derby winnings to Tunnel for Towers, a non-profit organization that aids the families of fallen first responders.

“Being and living and being a part of the city on a day-to-day basis, and just kind of immersing myself in it, that’s when I felt like I started to understand it more and more,” said Alonso, who was a six-year-old in Florida at the time of the attacks. “There are countless amounts of families, friends, co-workers -- there are so many people who are affected by this.”

The Yankees had a 4:05 p.m. ET game on Friday, and thus were the first to take the field wearing first responder caps. Starting pitcher Gerrit Cole came to the mound wearing an NYPD cap, alongside teammates wearing both NYPD and FDNY gear.

Yankees manager Aaron Boone, general manager Brian Cashman and outfielder Brett Gardner laid a wreath at the 9/11 Monument in Yankee Stadium’s Monument Park on Friday afternoon, honoring the men and women who lost their lives in the terrorist attacks.

“It was an awful day in our country, one of those moments you certainly remember where you were,” said Boone, who was in Chicago playing for the Reds at the time. “What stands out is the amazing World Series that year, and also the Mets and Braves game, the first day back -- how emotional that was for not just the city of New York, but I think our entire nation.”

Alonso, who grew up idolizing Piazza, has watched video of the catcher’s go-ahead homer against the Braves on Sept. 21, 2001 “so many times.” That game, at Shea Stadium, was the first professional sporting event in New York City following the attacks.

“It was like the official marker of the healing process,” Alonso said.

Now that MLB has approved the first responder caps for in-game use, Alonso’s hope is that wearing them will become an annual Sept. 11 tradition. Next year, the Mets will play the Yankees at Citi Field on the 20th anniversary of the attacks. Alonso would like both teams to wear caps recognizing the heroes of that day.

“I don’t know if it’s going to stand,” Alonso said. “But I hope that every year it does. And it should. I feel like it should.”