For some reason (a while ago) I decided to look up the history of the song, “I’m Dreaming of a White Christmas”. I think it was after hearing the song and thinking of this XKCD comic:

There’s more to say about this which is not merely about cultural time period or generation, but that’s for another blog post.
White Christmas is itself a nostalgic song which is, I think, part of what connected the two in my mind. It was first published in 1941 (and made famous in the 1942 movie Holiday Inn) but was written a year or two earlier by Irving Berlin. One story is that it was while he was in a hotel in California. This is supported by the (typically omitted) first verse:
The sun is shining, the grass is green,
The orange and palm trees sway.
There’s never been such a day
in Beverly Hills, L.A.
But it’s December the twenty-fourth,—
And I am longing to be up North—
Which brings up the question of Berlin’s history. It turns out that he was a Russian Jew, born Израиль Моисеевич Бейлин (Roughly, Israel Moiseyevich Beileen) in Imperial Russia in 1888 (or 1889—records weren’t as good those days). His family emigrated to the United States in 1893, when his name was changed to Israel Baline. (In 1906, when his first song was published, his name was misspelled as “I. Berlin,” and apparently it stuck. Presumably at some point he changed his name to Irving to avoid anti-Jewish prejudice, but I don’t know.)
Berlin’s family stayed in New York City for years before Irving became a success and started visiting other parts of the country. Thus his youthful memories would be of treetops that “glisten in the snow” in strong contrast to the California treetops which look the same in winter as in every other month.
Which is, of course, quite reasonable—I’ve heard that southern California’s lack of seasons takes some getting used to. The funny thing is that this doesn’t really translate out of the context of someone from New York who’s moved to California (or other southern parts of the US). I was one such person who didn’t have this experience; I grew up in the north-east of the United States in a similar sort of climate to where Berlin did, though around seventy years later. I’d heard the song growing up but didn’t know its context, so I adapted it to the context I knew.
Specifically, to the context where it was snowy on Christmas only one out of every 5 (or so) years. Hence in anticipation of Christmas (when Christmas songs are generally played in the United States) one could dream of a white Christmas like the ones one remembers from years gone by. But this is a very different thing because even a snowless Christmas in the north-eastern United States is still cold and largely dark (happening as it does at most about 5 days after the shortest day of the year). It’s still at a time of year that feels very different to spring-through-fall and still a counter-point to the bleakness of the season.
Thus for me it’s a sort of grass-is-greener wishing for things to be optimal instead of highly different. (If you come from a part of the world that never gets much snow, see my post Snow Is Peaceful for why a white Christmas is nice.) This is a picky sort of nostalgia of which the Boomers are often accused, though in this case unfairly since the song was written much closer to the beginning of the war rather than to the end of it, and by someone who was born in the previous century.
Even saying that this song was part of the Boomer’s childhood, as the XKCD comic does, is not very accurate. Leaving aside the reasonableness of the demarcations of the “baby boom” and using the most expansive definition of it, the Baby Boomers were born from 1945-1964. Assuming that people start forming life-long memories at about the age of 7 this means that White Christmas had been playing for at least ten years before any baby boomers would have remembered it from their childhoods. If the average boomer was born in 1954, the song had been popular for about 19 years before they would remember it from their childhood. And it should be noted that it isn’t seven-year-olds who put songs onto the charts. White Christmas is not a centuries-old tradition but it was clearly an earlier tradition than the childhood of the baby boomers.
(Incidentally, I can’t begin to imagine what this song sounds like to someone who grew up in the southern hemisphere of the world since for them Christmas takes place in the summer and you’d have to live on a mountain or in Antarctica to have snow on Christmas. In fact, in most places in the southern hemisphere I suspect a white Christmas would be an epic disaster since it would mean the death of most of their food crops and probably a good chunk of their livestock.)