"I had work yesterday."
The topic is きのう, meaning yesterday. It's followed by は, the topic marker. Then comes しごと, or work. Finally, the normal copula です is replaced with でした because it's past tense. So a literal word-by-word translation could be, "Regarding yesterday, party did." The one who partied is implied to be the speaker in the absense of context, but that depends on the conversation or paragraph.