" working day of this kind, which is formed by the succession of more or less numerous interrelated working days, I call a working period. If we speak of the working day, then we mean the length of time for which the worker must daily expend his labour-power, must work. If we speak of the working period, on the other hand, this means the number of inter-related working days that are required, in a "

― Karl Marx , Capital: A Critique of Political Economy, Volume 2