Symbolism, Imagery, Allegory
The working day is a pretty straightforward concept. Today, we just call it the workday. It's the amount of time you have to go to work.
Now, think of how, for example, the workweek is usually supposed to be 40 hours long: 8 hours a day, five days a week. There's no law of nature making life be this way. It's the outcome of class struggle between capitalists, who want as long of a working day as possible, and laborers, who want the working day to be short.
The working day is divided into necessary labor and surplus labor. Necessary labor is the amount of time it takes to produce the value of the means of sustenance—the commodities that keep workers alive. Everyone needs food, clothing, and shelter to survive, and those things have a cost. The amount of labor required to produce them is the necessary labor time. Anything above that, which goes into the capitalists' pockets, is surplus labor.
You can debate what precisely constitutes the means of sustenance—do people really need television to stay alive?—but even if you want to be generous and say workers need such-and-such luxuries to stay alive, the gist of the idea still stays the same.
The class struggle over reducing the length of the working day in England in the 19th century is recounted in detail by Marx in chapter 10. See the "Plot Analysis" for a look at how that struggle went down.