JSFiddle - React, Tailwind, and code Playground

HTML

<link rel="stylesheet" href="http://ajax.googleapis.com/ajax/libs/jqueryui/1.8.17/themes/base/jquery-ui.css">
<p class="datepair" data-language="javascript">
 <label>Day<em>*</em></label>
 <input type="text" id="date_o" placeholder="Please select date" class="date start" />
</p>

JavaScript

$("#date_o").datepicker({
    onSelect: function(dateText, inst) {
        var d = new Date(dateText);
        if(d.getDay() == 0){
            alert('You cannot choose sunday!');
            $('#date_o').val('');
        }
    }
});