Hi anirudhp,
This is not possible with Asp.Net CalendarExtender to select multiple date.
If you want to select multiple date use multiDatesPicker plugin.
https://www.c-sharpcorner.com/blogs/select-multiple-dates-in-jquery-date-picker1
HTML
Date:
<input name="txtMultiDatePicker" type="text" id="txtMultiDatePicker" />
<script type="text/javascript" src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<link rel="stylesheet" href="https://cdn.rawgit.com/dubrox/Multiple-Dates-Picker-for-jQuery-UI/master/jquery-ui.multidatespicker.css" />
<link rel="stylesheet" href="https://code.jquery.com/ui/1.12.1/themes/pepper-grinder/jquery-ui.css" />
<script type="text/javascript" src="https://code.jquery.com/ui/1.12.1/jquery-ui.min.js"></script>
<script type="text/javascript" src="https://cdn.rawgit.com/dubrox/Multiple-Dates-Picker-for-jQuery-UI/master/jquery-ui.multidatespicker.js"></script>
<script type="text/javascript">
$(function () {
$('#txtMultiDatePicker').multiDatesPicker();
});
</script>
Demo
OR
You can use two CalendarExtender to select multiple date.
https://forums.asp.net/t/1357129.aspx?AJAX+CalendarExtender+Can+user+select+multiple+dates+
