The Christmas Radio Times has belatedly appeared in my flat - this is the one time of year when I actually know what's going to be on TV. The rest of the time I like to treat broadcast schedules as roughly akin to the weather, and adjust my expectations, and possibly my clothing, depending on what turns up on the day. But Christmas is different: there might be a Humphrey Bogart season on in the middle of the nights on BBC4. That's not the kind of thing I'm prepared to leave to chance. And quite aside from the noir, if I don't do my research, I could easily miss a brilliant Christmas film. Were it not for the Radio Times a few years ago, I might still never have seen The Bishop's Wife, and I wouldn't know that Cary Grant had even made a Christmas film, let alone one of my favourites.

What makes a film a Christmas movie isn't always easy to define. Obviously, there are films which are about Christmas, like Miracle on 34th Street, in which Kris Kringle has to prove to the satisfaction of a judge that he's really Father Christmas, and not just a department store Santa. It's a great movie because it crosses over two seemingly disparate genres: the children's film, and the courtroom drama. In doing so, it reminds us that Christmas is about rediscovering a childlike faith which many of us had long since left behind.

But there are plenty of films which aren't at all about Christmas, but are set at Christmas. Gremlins, for example, is a Christmas favourite for many. For me, though, a true Christmas film doesn't have a scene where someone tells the story of her dad dressing up as Santa and breaking his neck coming down the chimney. And I can't feel Christmassy when there's so much mess. Someone has to clean up all that broken glass, you know. There's also When Harry Met Sally, which takes place over a succession of New Year's Eves, so it's hard to think of Meg Ryan and Billy Crystal without a Christmas tree lurking in the background somewhere.

Then there are the films which have nothing whatsoever to do with Christmas, except that we always watch them then. So half of the Harry Potter films, for example, have had cinema release dates in late November. The music is John Williams' at his most magical, and the images of steam trains, snow over the Hogwarts grounds, and plenty of candle-lit feasting all add to a festive feel.

Which is more than one can say for The Great Escape, which manages to be a Christmas film purely because, in the days before DVDs for Christmas and TV on demand, it was on every Christmas without fail, and you either watched it, or spent the afternoon talking to an uncle worse for wear on eggnog. The Great Escape isn't in the least bit festive, nor does it have a remotely happy ending, but it still reminds most of us of Christmas.

The classic Christmas stories are made anew for each generation. The recent Jim Carrey version of A Christmas Carol is nothing if not traditional. This story has been remade so many times that the Internet Movie Database lists Scrooge appearing 73 times (not counting Scrooge McDuck); the oldest surviving version dates back to 1910. And A Christmas Carol in virtually any form is usually a winner (my favourite is The Muppets' Christmas Carol, because I think Gonzo makes an excellent Dickens, and I like the singing cabbages): it reminds us of the spirit of Christmas, and it isn't preachy.

We often cherish most the films which we first saw as children, like It's A Wonderful Life. It's a far bleaker film than its saccharine title implies, and that is, of course, why it's brilliant. For many people, Christmas isn't a time of overflowing goodwill and joy, it's a time of pinched budgets and exhaustion, bordering on despair. James Stewart was always the perfect everyman, and George Bailey is just that: an ordinary man whose troubles drive him to the brink of suicide on Christmas Eve, until an angel steps in and shows him what the world would be like without him. David Niven and Loretta Young also need the help of Cary Grant's angel, Dudley, in The Bishop's Wife. Their woes aren't as all-encompassing as George Bailey's, but they are both in need of some help to remember that Christmas is more than another inconvenience in a full diary. Plus, there's an ice-skating scene, and Loretta Young gets a new hat. I count that as a Christmas win.

So re-watching films that were made in the 40s (Miracle on 34th St, It's A Wonderful Life and The Bishop's Wife all came out in the two years following the end of WW2: no wonder they're so determined to be life-affirming) is part of a Christmas ritual. Repetition is integral to the way most of us celebrate Christmas, after all: we eat the same meal, with the same people, curl up on the same sofa year after year.

But there's more to it than that. Ultimately, Christmas films offer us a vision of how we think Christmas should be: thick snow blanketing homes, as we're ensconced with our loved ones, exchanging humble, longed-for gifts, and playing games after which the losing teams don't cry. The reality, of course, is that snow at Christmas would be a disaster, now that so many of us live a long drive away from our loved ones. Even snow in the run-up to Christmas is pretty grim - this week has proved that. We all have groceries to buy and last-minute presents to track down: we don't have time for snow. Dreaming of a white Christmas means dreaming of traffic jams, power cuts and fractured wrists. So let's watch White Christmas on TV, but hope for a grey one in life.