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The blue cells in the map signify market gains, while the orange cells indicate losses. White boxes are dates that don’t exist or fixed-date holidays when the market is closed, like Christmas and New Year’s Day. The table on the left shows the percentage of time a given day increases, while the table on the right shows the median percent change for each date.
A few trends emerge. Apr. 15, tax day, has been a particularly good day for the stock market historically, as are some dates around national holidays: Jul. 3, Dec. 24 and 26, and Jan. 2. The worst days include Apr. 7 and Feb. 29 (which only occurs every four years, and thus has less data overall).
Kadish notes that this graph is for entertainment purposes only, not for creating investment strategies -- obviously the stock market's day-to-day performance can vary significantly from the historical average.