When the characters representing various seasons get along with each other, face off against each other or interact in other ways. Typically they include:

Lily White as Spring
Kazami Yuuka as Summer, even though she has no relation to this season in canon
Aki Minoriko and Aki Shizuha as Fall
Letty Whiterock and Cirno as Winter, even though Cirno, like Yuuka, has no relation to this season in canon

See also pool #532 and pool #1073, comics using this theme as key points to their stories.