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There are similar year-to-year variations in the number of weekends, with the result that the number of working days can also be very variable (see chart below). The different weighing methods produce different numbers (an average of 259.1 days on a population-weighted basis, 251.4 for GNP and 251.6 for stock market capitalization) because many countries with below-average GNP work a six day week.
This gives rise to an interesting economic question: Should GNP be higher in years when there are more working days? Not being economists, we're not qualified to answer that question. However, we can point out that the effect is very significant in some years. For example, in 2032 there will be 0.9% more working days than in 2031 (see chart below).