River of Fireflies (1987)

During the winter in Toyama in 1962, Tatsuo Mizushima is fretting about his feelings for his classmate Eiko Tsujisawa while agonizing about his upcoming high school examinations. When he was little, he and Eiko had heard that in April of a year when the winter has been very harsh, any couple finding fireflies together needs to get married. However, his family's situation does not allow Tatsuo to continue his worrying about such things for very much longer.