计算黑色星期五：在西方，星期五和数字13都代表着坏运气，两个不幸的个体最后结合成超级不幸的一天。所以，不管哪个月的十三日又恰逢星期五就叫“黑色星期五”-Calculation on black Friday: in the west, Friday and number 13 stands for bad luck, finally combined into two unfortunate individual super unlucky day. So, no matter which mo