WHEN HE FIRST wrote the song in 1939, Irving Berlin W thought so little of it that he tossed it into a trunk. Three years later, the composer described by Jerome Kern as "Berlin is American music" pulled it out to fit into a Bing Crosby-Fred Astaire movie called "Holiday Inn." Crosby, who seldom waxed enthusiastically about any tune and who didn't want to sing the song in the first place because it might commercialize the most important day in his Catholic church, ended up singing it melifluously: "I'm dreaming of a White Christmas/ With every Christmas card I write/ May your days be merry and bright/ And may all your Christmases be white."

To American soldiers fighting World War II in distant lands, the song was a highly nostalgic reminder of home and the holiday season. "It happened to hit the public at a time when American boys were all over the world," Berlin once said. "People interpreted it as a song of a homesick soldier -- which I hadn't written at all. A peace song became a top war song. The public read things into it I never intended. Of course, I'm thankful they did."

"White Christmas" became a pop anthem for this century, as inextricably tied to the season as "Silent Night." Crosby's version, which is almost as popular today as it was in the '40s, has sold more than 25 million copies. Overall, the song has sold 136,260,000 copies in more than 550 versions, according to Berlin's publishing company. Almost 6 million copies of sheet music have been sold. And that's in the Unites States and Canada only.

"Holiday Inn" had been an original idea of Berlin's, dealing with a man who opened a hotel only on holidays, preferring to farm the rest of the year. "White Christmas," of course, was one of those holidays. Crosby, who recorded the song in 18 minutes, recalled its impact during the war. "So many young people were away and they'd hear this song and it would really affect them. I sang it many times in Europe in the field for the soldiers. They'd holler for it; they'd demand it and I'd sing it and they'd all cry. It was really sad."

Originally, the opening line had been "I'm sitting here in Beverly Hills, dreaming of a white Christmas." Berlin was loathe to change the line and did so only after being convinced by Decca Records founder Jack Kapp that the line would have little meaning to the rest of the world. Crosby remembered the day Berlin first sang the song for him. "He thought it was a nice little song, or -- as he said -- 'I have an amusing little number here.' When he demonstrates a song before a picture for the cast, director and everybody -- you have to hug him to hear him. He Berlin has tremendous enthusiasm, but a tiny little voice and he plays a kind of broken-down piano."

"White Christmas" reappeared three years later in another Crosby-Astaire film, "Blue Skies," which was little more than an anthology of Berlin songs (30 of them, in fact). In 1954, it provided the title for the most successful Berlin picture, this time teaming Crosby and Danny Kaye. NBC paid $1.5 million, an incredibly high figure for that era, to run it over their network once a year for three years, with an option for three more telecasts at $700,000 a year.

And, of course, "White Christmas" was always the last thing on Crosby's specials. He seldom let anyone sing it with him; it was his song, his and Mr. Berlin's.