Irving Berlin was born in Russia.His parents emigrated to the US when he was five.It was Russian white Christmases that they remembered. Thousands and thousands of immigrants had the same memory of their old homelands. Those Christmases were " just like the ones I used to know ". That's nostalgia, in its original meaning: homesickness, fond memory of home. Imagine the parents and the grandparents saying to the children "Yes, it's Christmas but it's not like Christmas used to be, back home.You cannot imagine how...."[and so on repeatedly ad nauseam] !