calender code c#

i have a calender code which work out but if click button ( ... ) , calender pop out. Then have to choose date.  the calender will close. after that if i click again on click (...) button the calender pop out If i didn't choose any date means the calender cannot close. so again i have to choose date.  If i click any other place also the calender cannot be close.  So i need some code which can close the calender without choose date if i already choose date before. the calender should close if click any other place. How to do that in c#. Below i attach with calender code. I hope any 1 can help me out. Advance thanksss..

Calender code in c#
using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;

public partial class Default : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
...