Today most people in the United States put up their Christmas trees in early December, a time corresponding with what Christians would call "Advent". Some even put their trees up as early as the day after Thanksgiving or even earlier (although some of their neighbours might look at them oddly). That having been said, there was a time when the traditional time for putting up one's Christmas tree was Christmas Eve or, at least, the afternoon of the day before Christmas. Indeed, this is reflected in many of the classic holiday films of the 1940s.
While many of the classic Yuletide movies of the Forties have trees being trimmed on Christmas Eve, there are a few in which the event takes place a slightly earlier date. It would seem the late Forties was a time of transition, when many still put their Christmas trees up on Christmas Eve, but some were beginning to put them up earlier.
An example of a film in which the Christmas tree is trimmed on Christmas Eve proper is the 1944 classic Christmas in Connecticut. In the film cooking writer Elizabeth Lane (Barbara Stanwyck) is coerced by her publisher (Sydney Greenstreet) in hosting a returning war hero, Jefferson Jones (Dennis Morgan), for Christmas dinner. Among the things that Miss Lane and Mr. Jones do on Christmas Eve is set up and trim the Christmas tree.
The classic It's a Wonderful Life was released a few years after Christmas in Connecticut in 1946, but the Baileys still trimmed their Christmas tree on Christmas Eve. In fact, that Christmas Eve is also George Bailey's "crucial night", part of what might be the most famous climax in any Yuletide movie ever made.
The Bishop's Wife, released only a year after It's a Wonderful Life in 1947, shows that the time people set up their Christmas trees was beginning to change in the United States. At least several days before Christmas Eve, or perhaps more, Professor Wutheridge (Monty Woolley) buys a tiny Christmas tree. When the bishop's wife of the title, Julia (Loretta Young), and the angel Dudley visit the Professor, he already has the Christmas tree set up and decorated in his home. While the Professor already has his Christmas tree set up well before Christmas Eve, however, Bishop Brougham and his wife Julia don't have their tree set up and trimmed until Christmas Eve (well, actually, it is Dudley who does the trimming...). From The Bishop's Wife it would seem that some people were already setting up their trees before Christmas Eve, while others were still doing it on the traditional date.
It is in a film released the same year as The Bishop's Wife that a Christmas tree is shown being trimmed before Christmas Eve. In It Happened on Fifth Avenue it would appear that the Christmas tree is set up and trimmed a few days before Christmas Eve, or at least a day before the traditional date. Like the Professor in The Bishop's Wife, then, this shows that people were trimming their trees at least a little bit before Christmas Eve.
While It Happened on 5th Avenue has the Christmas tree erected not long before Christmas Eve, it seems possible that in Holiday Affair (1949) the Christmas tree was set up and trimmed as much as two weeks before Christmas Eve. At the very least the Christmas tree is in place and decorated several days before Christmas Eve. This perhaps shows that towards the end of the decade many Americans were trimming their trees well before Christmas Eve, at times much more in keeping with modern custom.
It is difficult to say what precipitated the change in when Americans set up their Christmas tree, although if the movies are any indication it would seem to have begun in the late Forties at least. Of course, if the movies are also any indication, the practice of trimming the tree on Christmas Eve appears to have persisted into the Fifties and Sixties. In The Apartment (1960) on Christmas Eve, Sheldrake (Fred MacMurray) tells Miss Kubelik (Shirley MacLaine) that he has to get home and trim the tree. Regardless, the transition appears to have been complete by the Eighties. In National Lampoon's Christmas Vacation (1989), Clark Griswold (Chevy Chase) buys a tree and sets it up a few weeks before Christmas (probably late November/early December).
Regardless of why the date most Americans trim their Christmas trees changed, the change was certainly reflected in American movies. What is more, it would seem that the Forties was the decade in which the change began to take place. After all, the tree is trimmed on Christmas Eve in Christmas in Connecticut in 1944, but by 1949 the tree is trimmed many days earlier in Holiday Affair.