Fifteen years after a fractured heart began to heal a tiny bit around Christmas, the holiday celebrated around the world remains tough on the son Billy Martin left behind 25 years ago this coming Thursday.

“I spend a lot of the day thinking about him,’’ Billy Martin Jr. told The Post from his Arlington, Texas, home. “Even when my kids are really into their joy, it’s sad for me because they never met him and he was amazing with kids.’’

Billy III, 16, Gunner Mantle, 15, and Olivia Grace, 12, are the grandchildren Martin never met because of a 1989 Christmas Day auto accident that killed the fiery former Yankees second baseman and five-time manager who led the club to the 1977 World Series title.

Longtime friend Billy Reedy, a Detroit tavern owner near Tiger Stadium who befriended Martin when he managed the Tigers, and Martin were drinking Christmas Day in a Fenton, N.Y., bar northeast of Binghamton before Martin’s truck hit a patch of ice and went down a 300-foot embankment 25 yards from Martin’s house.

Reedy acknowledge the pair were drinking but said the ice and no guardrails on the road played a bigger part in Martin’s death.

Reedy told police he was driving to keep his buddy from a DUI bust, but Martin was dead from a broken neck two feet away. Reedy suffered a broken hip, and his lie resulted in a DUI charge.

Though Martin’s grandkids never met him, they have a nickname for him that was born out of the No. 1 on the back of the Yankees uniform that is retired.

“They still call him ‘1 Pa’ and he would have loved to hear that,’’ said Billy Jr., who recently turned 50.

Martin was 61 when he perished and didn’t believe he was done managing the Yankees.

At the Winter Meetings in Nashville weeks before he died, Martin told Michael Kay, then a Yankees beat writer, that owner George Steinbrenner was going to bring him back to replace Bucky Dent, who took over for Dallas Green during the 1989 season.

Dent eventually was fired during the 1990 season and replaced by Stump Merrill.

When that story surfaced in January 1990, Steinbrenner refuted it, but Martin’s widow, Jill, said her husband was told that was going to happen.

Billy Jr. and Reedy said it took 10 years for Christmas to represent more than heartbreak. Billy Jr. said it was having Billy III around. Reedy said the birth of his granddaughter Mikka did the trick.

Reedy, who told The Post in 1999 that he hadn’t a drink since the wreck, died from pancreatic cancer in July 2009. He was 72.

Despite firing Martin several times, Steinbrenner had a special feeling for him. According to Hal Steinbrenner, The Boss was crushed to hear the awful Christmas news 25 years ago.

“I remember vividly,’’ Hal Steinbrenner wrote in an email. “Billy had just been in Tampa to participate in the Christmas concert that we do every year for kids in Hillsborough and Pinellas counties. He read ‘Twas the Night Before Christmas.’ We all enjoyed seeing him as he was like family. My dad and I were at the Bay Harbor Inn watching football the day he died. I was in the restaurant when I heard, so I went up to tell him. He already knew and was visibly distraught. No doubt about it. People don’t realize what a special relationship they had.’’

For Willie Randolph, a second baseman like Martin, the memory of his former manager hasn’t faded.

“He has a special place in my heart, and I think about him a lot,’’ Randolph said. “His death hurt me because we had a very good relationship. Billy and I were really close. He was my first manager and he took me under his wing. There were players who didn’t like Billy, but he was good to me.’’

A quarter of a century ago, Randolph wasn’t totally surprised Martin died the way he did, but that didn’t make it any easier to handle.

“I wasn’t surprised, but it was tragic. I knew how Billy was always in the middle of something,’’ Randolph said. “I was there through all the turmoil and felt he would self-destruct. I wasn’t surprised or shocked, but it hurt. A traffic accident, you had a feeling it was going to end with something like that. I saw him fight and get beat up. At some point you hoped he would slow down and settle down.’’

Lou Piniella, who replaced Martin as manager in 1988, recalled Christmas 25 years ago.

“We were skiing in Vermont, my family, and we got the call. It was a terrible day,’’ Piniella said from his Tampa home. “The way it happened was a sobering moment for us. Twenty-five years, that shows you how quickly time passes. I remember the phone call like it was yesterday.’’