<asp:calendar id="Calendar1" runat="server" xmlns:asp="#unknown">
      onselectionchanged="Calendar1_SelectionChanged"></asp:calendar>

protected void Calendar1_SelectionChanged(object sender, EventArgs e)
       {
           var dt = Calendar1.SelectedDate.ToShortDateString();
           TextBox1.Text = Convert.ToString(dt);

       }

在日历控制的Day_Render事件中编写代码。工作正常。

Write the code in Day_Render event of calender control.Working fine.

protected void Calendar1_DayRender(object sender, DayRenderEventArgs e)
    {
        TextBox1.Text = Calendar1.SelectedDate.ToString();
    }