Any excuse will do to show "Casablanca."

Repetory cinema, which is what we call old movies these days, is packaged in "festivals" by "themes." This makes it sound as if you are taking a mid-winter vacation in Cannes if you merely decide, for one evening, to go back to a favorite old flick rathr than risk seeing one of the 1978 Christmas releases that will be occupying the first-run theaters all January.

But whatever the theme, you get at least two proven Bogies, "Casablanca" and "To Have and Have Not," plus some good movies you missed o want to see again, and some that perhaps you missed on purpose the first tiem around.

The Biograph, which is having a "Love on the Silver Screen" festival, is showing "Cassablanc" and "To Have and Have Not" February 16 through 20, while the American Film Institute Theater, which is having a "The War Years" festival, is showing "Casablanca" at midnight this Fiday and Saturday and "To Have and Have Not" February 6 at 8:30 and February 7 at 8:45.

Other varieties of silver-screen love as defined by the Biograph festival include three-way love in "Jules and Jim," this Friday through Sunday; unrequited love, subdivision of deranged, in "Story of Adele H" cross-generational love, in "Lolita" and "Pretty Baby"; and unhappily-ever-after love, classics division, s in "Anna Karenina" and "Romeo and Juliet."