(int)DateTime.Now.DayOfWeek

// Summary:
//     Specifies the day of the week.
[Serializable]
[ComVisible(true)]
public enum DayOfWeek
{
    // Summary:
    //     Indicates Sunday.
    Sunday = 0,
    //
    // Summary:
    //     Indicates Monday.
    Monday = 1,
    //
    // Summary:
    //     Indicates Tuesday.
    Tuesday = 2,
    //
    // Summary:
    //     Indicates Wednesday.
    Wednesday = 3,
    //
    // Summary:
    //     Indicates Thursday.
    Thursday = 4,
    //
    // Summary:
    //     Indicates Friday.
    Friday = 5,
    //
    // Summary:
    //     Indicates Saturday.
    Saturday = 6,
}

CultureInfo _culture = (CultureInfo)CultureInfo.CurrentCulture.Clone();
CultureInfo _uiculture = (CultureInfo)CultureInfo.CurrentUICulture.Clone();

_culture.DateTimeFormat.FirstDayOfWeek = DayOfWeek.Saturday;
_uiculture.DateTimeFormat.FirstDayOfWeek = DayOfWeek.Saturday;

System.Threading.Thread.CurrentThread.CurrentCulture = _culture;
System.Threading.Thread.CurrentThread.CurrentUICulture = _uiculture;

Thread.CurrentThread.CurrentCulture = New CultureInfo("ar-EG")

DateTime.Now.DayOfWeek; // returns Sunday
(int)DateTime.Now.DayOfWeek; // returns 0
DateTime.Now.DayOfWeek(DayOfWeek.Monday); // returns 6

public static class ExtensionMethods
{
    /// <summary>
    /// Returns an zero-based index where firstDayOfWeek = 0 and lastDayOfWeek = 6
    /// </summary>
    /// <param name="value"></param>
    /// <param name="firstDayOfWeek"></param>
    /// <returns>int between 0 and 6</returns>
    public static int DayOfWeek(this DateTime value, DayOfWeek firstDayOfWeek)
    {
        var idx = 7 + (int)value.DayOfWeek - (int)firstDayOfWeek;
        if (idx > 6) // week ends at 6, because Enum.DayOfWeek is zero-based
        {
            idx -= 7;
        }
        return idx;
    }
}