「曜日 (0:日曜 〜 6:土曜)」
= (y + [y/4] - [y/100] + [y/400] + [2.6m + 1.6] + d) % 7

=(1990+1990/4-1990/100+1990/400+2.6*2+1.6+3)/7=3 sui do