When should you use the imperfect rather than the pasado compuesto?

The imperfect tense is used to describe things that were ongoing in the past for example:Cuando era pequeña, jugaba al tenis todos los sábados.Whereas, the Pasado Compuesto is used to describe an action that only occurred once at a specific time, for example:El sábado pasado, yo he comprado una pelota.