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Market Hours and Valuations
Company values (and thus stock prices) rely on a much larger time frame than "a weekend".  First, markets are not efficient enough to know what a companies sales were over the past 2-3 days (many companies do not even know that for several weeks).  They look at performance over quarters and years to determine the "value" of a company.  They also look forward, not backwards to determine value.  Prior performance only gives a hint of what future performance may be.  If a company shut its doors over a weekend and did no sales, it still would have value based on its future ability to earn profits.