在DateTimePicker控件中更改周显示

您可以使用SendMessage将MCM_SETFIRSTDAYOFWEEK消息发送到DateTimePicker的MonthCalendar（在文档中阅读与此消息有关的注释）。

您首先发送一条DTM_GETMONTHCAL消息以获取MonthCalendar控件的句柄。如图所示，您可以DropDown在DateTimePicker的事件处理程序中发送此消息。
如果句柄有效，则设置发送的第一天MCM_SETFIRSTDAYOFWEEK。
该lParam值确定MonthCalendar中显示的第一天：

0=星期一，1=周二，依此类推。

如果要构建自定义控件（最好是IMO），则可以在此处找到一个预构建的控件：
如何将DateTimePicker下拉列表设置为仅选择Years或Months？

它显示了如何处理DateTimePicker及其MonthCalendar下拉菜单，以更改当前的View和类似的任务。

internal const int DTM_FIRST = 0x1000;
internal const int DTM_GETMONTHCAL = DTM_FIRST + 8;
internal const int MCM_FIRST = 0x1000;
internal const int MCM_SETFIRSTDAYOFWEEK = MCM_FIRST + 15;

internal enum MCWeekDay : int 
{
    Monday,
    Tuesday,
    Wednesday,
    Thursday,
    Friday,
    Saturday,
    Sunday
}

[DllImport("user32.dll", SetLastError = true, CharSet = CharSet.Auto)]
internal static extern IntPtr SendMessage(IntPtr hWnd, int Msg, int wParam, int lParam);

internal void MonthCalendarSetFirstDayOfWeek(IntPtr dtpHandle, MCWeekDay weekDay)
{
    IntPtr hWndCal = SendMessage(dtpHandle, DTM_GETMONTHCAL, 0, 0);
    if (hWndCal != IntPtr.Zero) {
        SendMessage(hWndCal, MCM_SETFIRSTDAYOFWEEK, 0, (int)weekDay);
    }
}

private void dateTimePicker1_DropDown(object sender, EventArgs e)
{
    MonthCalendarSetFirstDayOfWeek((sender as Control).Handle, MCWeekDay.Tuesday);
}

VB.Net版本：

Imports System.Runtime.InteropServices

Friend Const DTM_FIRST As Integer = &H1000
Friend Const DTM_GETMONTHCAL As Integer = DTM_FIRST + 8
Friend Const MCM_FIRST As Integer = &H1000
Friend Const MCM_SETFIRSTDAYOFWEEK As Integer = MCM_FIRST + 15

Friend Enum MCWeekDay As Integer
    Monday
    Tuesday
    Wednesday
    Thursday
    Friday
    Saturday
    Sunday
End Enum

<DllImport("user32.dll", SetLastError:=True, CharSet:=CharSet.Auto)>
Friend Shared Function SendMessage(hWnd As IntPtr, Msg As Integer, wParam As Integer, lParam As Integer) As IntPtr
End Function

Friend Sub MonthCalendarSetFirstDayOfWeek(dtpHandle As IntPtr, weekDay As MCWeekDay)
    Dim hWndCal As IntPtr = SendMessage(dtpHandle, DTM_GETMONTHCAL, 0, 0)
    If hWndCal <> IntPtr.Zero Then
        SendMessage(hWndCal, MCM_SETFIRSTDAYOFWEEK, 0, weekDay)
    End If
End Sub

Private Sub DateTimePicker1_DropDown(sender As Object, e As EventArgs) Handles DateTimePicker1.DropDown
    MonthCalendarSetFirstDayOfWeek(DirectCast(sender, Control).Handle, MCWeekDay.Tuesday)
End Sub