Use jquery datepicker. Its very light weight and easy to integrate and use in you ASP.NET application.
Add following javascript.
<link rel="stylesheet" href="http://code.jquery.com/ui/1.10.2/themes/smoothness/jquery-ui.css" />
<script src="http://code.jquery.com/jquery-1.9.1.js"></script>
<script src="http://code.jquery.com/ui/1.10.2/jquery-ui.js"></script>
<link rel="stylesheet" href="/resources/demos/style.css" />
<script type="text/javascript">
$(function () {
$("#<%=txtFromDate.ClientID %>").datepicker({ dateFormat: 'M dd, yy' });
});
</script>
Then add Textbox that is to be used as calendar.
<asp:textbox id="txtFromDate" runat="server" width="150px" xmlns:asp="#unknown"></asp:textbox>
On server side, you will easily get the selected date as
txtFromDate.Text
.