년/월/일 로부터 요일 구하기

W = (k + floor(2.6m - 0.2) - 2C + Y + floor(Y/4) + floor(C/4)) mod 7                   [1]

k is day (1 to 31)
m is month (1 = March, ..., 10 = December, 11 = Jan, 12 = Feb) Treat Jan & Feb as months of the preceding year
C is century (1987 has C = 19)
Y is year (1987 has Y = 87 except Y = 86 for Jan & Feb)
W is week day (0 = Sunday, ..., 6 = Saturday)


이걸 이용해서 잘 해 보면 다음과 같이 c 코드를 만들 수 있다.
// getting day of week, given date (yyyy.mm.dd) // 0 is Sunday, 1 is Monday ... int32 dayFromDate(int y, int m, int d) { int32 c; if (m < 3) { y--; m += 10; } else { m -= 2; } c = y / 100; y = y % 100; return (d + (26*m-2)/10 - 2*c + y + y/4 + c/4) % 7; }


reference :
[1] How to determine the day of the week, given the month, day and year
http://www.mcs.csueastbay.edu/~malek/Mathlinks/Weekdays.html