This example assumes the main page from which users will pop up the calendar is named Calender.aspx. The following procedure shows how to create the popup calendar:

1. In Calendar.aspx, add these controls:

LinkButton

2. Create a TextBox to display the selected date:
3. Place the following JavaScript function in the Calendar.aspx page:
4. Create a new .aspx page named DatePicker.aspx. This is the page that pops up when users click the calendar LinkButton.
5. Place an ASP.NET Calendar control on this page.
6. Put the following code in the Calendar’s DayRender event handler:

// C#
protected void Calendar1_DayRender(
   object sender, DayRenderEventArgs e)
{

   HyperLink hlnk = new HyperLink();
   hlnk.Text = ((LiteralControl)e.Cell.Controls[0]).Text;
   hlnk.Attributes.Add("href", "javascript:SetDate('" +
      e.Day.Date.ToShortDateString() + "')");
   e.Cell.Controls.Clear();
   e.Cell.Controls.Add(hlnk);
}

// VB
Protected Sub DatePicker_DayRender(
   ByVal sender As Object, ByVal e As
   System.Web.UI.WebControls.DayRenderEventArgs)
   Handles DatePicker.DayRender
   Dim hl As New HyperLink()
   hl.Text = CType(e.Cell.Controls(0), _
      LiteralControl).Text
   hl.NavigateUrl = "javascript:SetDate( _
      '" & e.Day.Date.ToShortDateString() & "');"
   e.Cell.Controls.Clear()
   e.Cell.Controls.Add(hl)
End Sub

7. Copy the following JavaScript function into DatePicker.aspx.

At this point, you should be able to click the button to pop up the calendar. When users select a date, the calendar popup window will close, and the selected date will appear in the Textbox.

source :: devx