「先週」 (senshū) means "last week." This word refers to the week before the current one. It's often used with past tense verbs to describe actions that took place. For example, 「先週テストがありました。」(Senshū tesuto ga arimashita.) means "There was a test last week."

先週(せんしゅう)は前(まえ)の週(しゅう)。先週(せんしゅう)テストがありました。