Can さくじつ and きのう be used the same way? I know the terms mean yesterday. Thank you!
It depends on the context.
さくじつ and きのう both mean yesterday (and the same Kanji 昨日 is used for the words). A major difference in their usage is that さくじつ is almost always used in a formal context (written and spoken), while きのう is often used both in formal and informal contexts (written and spoken): Using さくじつ in a casual conversation seems weird. You can hear きのう not only in a casual conversation but also in the news.