本文為引用文章 僅作整理自用 原文連結： https://www.cnblogs.com/yxyl/p/9992841.html @網咖看壓力大

從週一到週日的順序，獲取排序數值：

int i = DateTime.Now.DayOfWeek - DayOfWeek.Monday;
if (i == -1) i = 6;

獲取某日起，星期一的日期：

public static DateTime GetMondayDate(DateTime someDate)
{
    int i = someDate.DayOfWeek - DayOfWeek.Monday;
    
 
if (i == -1) i = 6;// i值 > = 0 ，因為列舉原因，Sunday排在最前，此時Sunday-Monday=-1，必須+7=6。
    TimeSpan ts = new TimeSpan(i, 0, 0, 0);
    return someDate.Subtract(ts);
}

獲取當前時間上週六的時間：

public DateTime GetLastSaturday()
{
    DateTime time = DateTime.Now;
    if (time.DayOfWeek == DayOfWeek.Sunday)
    {
        time 
 
= time.AddDays(-2);
    }
    else
        time = time.AddDays(-1);
    while (time.DayOfWeek != DayOfWeek.Saturday)
    {
        time = time.AddDays(-1);
    }
    return time;
}