YESTERDAY'S SPRING is the telling of two disparate love affairs while chronicling the interlocking of the lives of two men and a woman during that half-dozen or so years that was the life span of the big band craze and a form of jazz called swing, that impoverished time just before and during World War II of crew-cuts, bobby soxers, ballrooms, hotel dancing, after-hour jam sessions, radio-remotes, convertibles, jitterbugs and the Selective Service Training and Service Act of 1940 . . . Though not a story about swing music or the big bands per se, the music and bands do play a large part (acting as a backdrop and pivot around which the story revolves).YESTERDAY'S SPRING is not another spin down nostalgia lane, not a valentine, but a sometimes erotic, hard-hitting, no-punches-pulled look at the way it really was right up to its explosive climax within the fiery confines of Boston's ill-fated Cocoanut Grove.