JSFiddle - React, Tailwind, and code Playground

CSS

.event a {
    background-color: #42B373 !important;
    background-image :none !important;
    color: #ffffff !important;
}

JavaScript

jQuery(document).ready(function() {
        
        // An array of dates
        var eventDates = {};
        eventDates[ new Date( '07/07/2015' )] = new Date( '07/07/2015' );
        
        // datepicker
        jQuery('#calendar').datepicker({
            beforeShowDay: function( date ) {
                var highlight = eventDates[date];
                if( highlight ) {
                     return [true, "event", highlight];
                } else {
                     return [true, '', ''];
                }
             }
        });
    });