This sentence is given in the imperfect tense. We know this because of the word entrenaba, which is the conjugation of the verb entrenar for the third person singular in the imperfect. Recall that the imperfect is used when talking about past actions, conditions, or events that occurred regularly or frequently or that were in progress in a moment in the past. So this sentence means:
<em>The swimming team used to train on Saturdays</em>
