Gramercy Park will open its doors to the public for one hour for caroling on Christmas Eve in what has become a yearly tradition.

The gates will be unlocked from 6 p.m. to 7 p.m. on Dec. 24 — rain or shine — for New Yorkers to enjoy the normally private greenery (including the 40-foot Norwegian Spruce) and some festive singing, said Arlene Harrison, president of the Gramercy Park Block Association and a trustee of the park.

“It’s important to open up the park once a year. It’s neighborly to do that,” said Harrison, who is also dubbed “the mayor” of Gramercy Park. “People love it, people from all over the country have been emailing me. It takes on a life on its own.”

The small park, between Park and 3rd avenues on 20th and 21st streets, is private and only residents living around it, and guests of the Gramercy Park Hotel, or members of one of the surrounding clubs or houses of worship can access keys to unlock the wrought iron gates.

Last year, Harrison said, about 1,000 people showed up for the event. It was the first year the event was advertised on social media.

But the tradition of caroling in the park has gone on for decades. Initially, it was put on for churchgoers of The Parish of Calvary-St. George’s, on the corner.

“When I became a trustee of the park, I wanted it to be for everybody,” she said. “It used to be 30 people standing in the snow, rain, cold and miserable.”

Now, Harrison puts up tents to protect the musicians, joking that she’s turned the event into “a nightmare” for area residents.

But if you want to experience the exclusivity of the park for even the hour it’s open, show up early.

The doors are unlocked at 6 p.m., Harrison said, but last year “people couldn’t even get in the park.”