Here's how it's implemented in Ruby:
static int
calc_wday(int year, int month, int day)
{
int a, y, m;
int wday;
a = (14 - month) / 12;
y = year + 4800 - a;
m = month + 12 * a - 3;
wday = day + (153*m+2)/5 + 365*y + y/4 - y/100 + y/400 + 2;
wday = wday % 7;
return wday;
}