Joni Mitchell in 1972. (AP)

Michael Ball, a British stage actor and singer, was watching a performance of traditional Christmas music at a London drama school several years ago when the students unwrapped Joni Mitchell’s “River.”

Ball said he was somewhat startled, given that the classic 1971 confessional isn’t really a Christmas song.

Never mind that its opening melody is “Jingle Bells” in a minor key and that the lyrics begin with a seasonal scene: “It’s coming on Christmas, they’re cutting down trees/They’re putting up reindeer, singing songs of joy and peace.”

Ultimately, “River” is a bereft song about a broken romance and a woman who desperately wants to escape her heartbreak, saying repeatedly: “I wish I had a river I could skate away on.” The despairing drama just happens to be set around the holidays.

“There were all these 18- and 19-year-olds doing traditional Christmas songs, and then, bang — they start doing ‘River,'” Ball recalled in a subsequent interview with The Washington Post. “I’m thinking: Where on Earth did this come from?”

Of course, Ball might have asked himself that same question: In 2000, he recorded a version of “River” for his holiday album, “Christmas.” At the time, he thought he was a maverick for placing the song alongside the likes of “Silent Night” and “Have Yourself a Merry Little Christmas.”

As it turns out, plenty of artists have been dreaming of a very, very blue Christmas over the last two decades: “River” — originally featured on Mitchell’s melancholy 1971 masterwork, “Blue” — has become a seasonal favorite, despite being “thoroughly depressing,” as Elbow frontman Guy Garvey noted at a 2009 Christmas concert.

Or perhaps it’s ascended to holiday-hit status precisely because it’s an antidote to all those “songs of joy and peace.”

“We needed a sad Christmas song, didn’t we?” Mitchell said with a chuckle on NPR in 2014. “In the ‘bah humbug’ of it all.”

“River” has long been a popular cover among musicians, more than 500 of whom have recorded it for commercial release. Countless others have performed it in concert.

But since British jazz-fusion guitarist Peter White featured it on his 1997 set, “Songs of the Season,” Mitchell’s composition has been included on dozens of Christmas albums, from Barry Manilow’s “A Christmas Gift of Love” and Tracey Thorn’s “Tinsel and Lights” to Sarah McLachlan’s Grammy-nominated “Wintersong” and Megan Hilty’s new “A Merry Little Christmas.”

“We kind of turned it into a Christmas song, even though it was not written as a Christmas song,” White, who started the “River”-as-holiday-song trend nearly two decades ago, said at a holiday concert in 2011.

When Idina Menzel was promoting her “Holiday Wishes” album in 2014, the “Frozen” star said she recorded a version of “River” because “I needed to make sure that I included those people that are feeling some kind of loss or loneliness, because the holidays aren’t holly, jolly Christmas all the time, you know?”

“It is a song I’ve grieved to, cried along with sung at the top of my voice too because it feels so good to do so,” British musician Beth Orton told the Wall Street Journal, when her “River” cover was included on Amazon.com’s misleadingly named holiday playlist, “All Is Bright.”

“I would dedicate this song to those who are grieving the loss of a sense of place, loved ones, family,” she said, adding: “I can’t imagine wanting to cover any other song related to Christmas (except ‘Little Drummer Boy,’ of course!).”

Still, James Taylor, who knows the song better than just about anybody aside from Mitchell, said in a 2006 interview with The Post that “I don’t know why it’s suddenly getting picked up as a Christmas song. But some things just become identified as seasonal songs, and this is now one of them.”

At the time, Taylor had just released “James Taylor at Christmas,” which included “River” — a song he’d first heard decades earlier, when Mitchell played it at her home in Los Angeles in 1970, shortly after it was written.

(Columbia Records) (Columbia Records)

“Most Christmas songs are light and shallow, but ‘River’ is a sad song,” Taylor told The Post. “It starts with a description of a commercially produced version of Christmas in Los Angeles . . . then juxtaposes it with this frozen river, which says, ‘Christmas here is bringing me down.’ It only mentions Christmas in the first verse. Then it’s, ‘Oh, I wish I had a river I could skate away on’ — wanting to fall into this landscape that she remembers.

“It’s such a beautiful thing, to turn away from the commercial mayhem that Christmas becomes and just breathe in some pine needles. It’s a really blue song.”

Which is exactly why Ball said he recorded it.

“I’m not a big fan of Christmas, and I think there are a lot of people who feel a bit melancholy at the holiday,” he told The Post. “We’ve all sort of been there: It’s coming on Christmas, all that preparation is going on, and you just want to escape. You don’t want to buy into it. It’s a time of year that brings up a lot of memories for people, and if you’re missing somebody, it’s hardest at this time of year.”

In some ways, it’s the perfect anti-Christmas song, running counter to the prevailing seasonal spirit.

And yet, various versions of “River” have become hits on the radio stations that play Christmas music and nothing else around the holiday, making Mitchell’s tune one of the rare “new” entries in the Christmas canon, alongside Mariah Carey’s “All I Want for Christmas Is You.”

“It’s the most beautiful, like, sad Christmas-time song I’ve ever heard in my life,” country singer Ashley Monroe recently told A.V. Club. “Every time I sing it I kind of cry a little on the inside.”

If Mitchell never intended to write a Christmas hit, she’d hardly be the first songwriter to find accidental seasonal success: According to Ace Collins, the author of “Stories Behind the Best-Loved Songs of Christmas,” multiple songs now considered Christmas classics weren’t actually intended for the holiday.

The most famous: “Jingle Bells,” which was written by one James Pierpoint for a Thanksgiving program at his Unitarian church in the 1850s.

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“River” has become popular in Hollywood, too, featuring in the seasonally set movie “Love Actually,” in a Christmas scene in “You’ve Got Mail,” and in shows from “Glee” and “Ally McBeal” to “E.R.,” which used McLachlan’s ethereal version of the song in a dark, holiday-themed episode.

“As much as we struggle to find different Christmas songs — things that aren’t ‘Let it snow, let it snow, let it snow’ — I’m sure artists go through the same thing when they’re making holiday albums,” Ann Kline, who was co-music supervisor for “E.R.,” once told The Post.

“There are only so many Christmas songs out there.”

Linda Ronstadt said “River” was an obvious choice when she recorded her own holiday album, “A Merry Little Christmas,” in 2000.

“I’d wanted to record it for years, and I just couldn’t figure out where to put it,” she told The Post in 2006. “I’d never heard it in the context of other Christmas songs, but I always wondered why that was.”

Ronstadt said at the time that she’d never discussed the meaning of the song with her old friend, Mitchell.

But she had some ideas about what might have inspired it — possibly including Mitchell’s daughter, who was born in the 1960s, when the singer was 21 and about to move from Saskatoon to Toronto.

Mitchell gave the child up for adoption and didn’t have any contact with her until 1997.

“I think that’s what a lot of her singing is about, because it has this very sad tinge,” Ronstadt speculated. “But who really knows if that’s what ‘River’ is about? The answer is: I don’t know, and I bet Joni doesn’t, either.”

For her part, Mitchell told NPR in 2014 that the song is about “taking personal responsibility for the failure of a relationship.”

“And my generation — you know, the ‘Me Generation’ — is known to be a Peter Pan, narcissistic generation, right?” she said. “So it’s really, you know — it’s really that aspect of our inability — you know, ‘I’m selfish and I’m sad.’ Right?

“You know, people think that’s confessional, but I’d say, you know, in my generation, you think that that’s a unique personal statement? You know what I mean? It’s like, no wonder there’s so many covers of it!”

Taylor said the song is most likely autobiographical, given that “it starts with a girl from Canada watching them try to make Christmas on La Brea in Los Angeles.” But he told The Post in 2006 that he’d never actually discussed the meaning with Mitchell, with whom he was romantically involved in the early 1970s.

“Do I want to know who she made cry, who she made say goodbye? Well, I haven’t asked her that question,” Taylor said. “That’s the only mystery in it: Who was it whose heart she broke?”

With a laugh, he added: “There were a lot of us.”

This post has been updated.

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