c#で六曜の計算

JapaneseLunisolarCalendarを使って計算すると便利です

static int RokuyoDate(DateTime clacDay)
{
    JapaneseLunisolarCalendar jpnOldDays = new JapaneseLunisolarCalendar();
    int oldMonth = jpnOldDays.GetMonth(clacDay); //旧暦の月を取得
    int oldDay   = jpnOldDays.GetDayOfMonth(clacDay);//旧暦の日を取得　

    var aaa = jpnOldDays.GetYear(clacDay);
    var bbb = jpnOldDays.GetEra(clacDay);

    //閏月を取得
    int uruMonth = jpnOldDays.GetLeapMonth(
            jpnOldDays.GetYear(clacDay),
            jpnOldDays.GetEra(clacDay));

    //閏月含む場合の月を補正
    if ((uruMonth > 0) && (oldMonth - uruMonth >= 0))
    {
        oldMonth = oldMonth - 1;              //旧暦月の補正
    }

    //ルールとしては
    // (月 + 日) % 6　の結果が六曜になり
    //大安→赤口→先勝→友引→先負→仏滅
    int rokuyo = (oldMonth + oldDay) % 6;
    return rokuyo;
}