This classic Christmas movie, which also screens on December 17th, 22nd and 28th, is part of the Big Screen Classics season at BFI Southbank. You can find full details here.
Chicago Reader review:
Vincente Minnelli created one of his masterpieces with this loosely plotted but tightly structured 1944 story of a middle-class family waiting through spring, summer, and fall for the opening of the Saint Louis World's Fair of 1904. One of the first films to integrate musical numbers into the plot, it explores, without condescension or simplemindedness, the feelings that drive the family members apart and then bring them back together again. And there's the sublime Minnellian spectacle of Judy Garland singing "The Trolley Song," "The Boy Next Door," and "Have Yourself a Merry Little Christmas." A great film.
Here (and above) is the trailer.