JSFiddle - React, Tailwind, and code Playground

HTML

<link rel="stylesheet" href="http://ajax.googleapis.com/ajax/libs/jqueryui/1.8.5/themes/cupertino/jquery-ui.css">
<input id="datepicker" />

JavaScript

var defaultDay = 16;
$('#datepicker').datepicker({
    beforeShowDay: function(date) {
        if (date.getDate() == defaultDay)
            return [1, 'ui-state-highlight ui-state-active'];
        return [1, ''];
    },

});