根据年月日计算星期几

蔡勒公式

算法实现：计算公元后的星期数

int century = year/100;
int yearw = year%100;
int monthw = month;
if(monthw <3){
	monthw += 10;
	yearw --;
}

int week = (century/4 - 2*century + yearw + yearw/4 + 13*(monthw+1)/5 + day - 1)%7;