jQuery UI Datepicker - Multiple Date Selections

When you modifiy it a little, it works regardless which dateFormat you have set.

$("#datepicker").datepicker({
                        dateFormat: "@", // Unix timestamp
                        onSelect: function(dateText, inst){
                            addOrRemoveDate(dateText);
                        },
                        beforeShowDay: function(date){
                            var gotDate = $.inArray($.datepicker.formatDate($(this).datepicker('option', 'dateFormat'), date), dates);
                            if (gotDate >= 0) {
                                return [false,"ui-state-highlight", "Event Name"];
                            }
                            return [true, ""];
                        }
                    });     

<div id="calendar"></div>
<script>
$(document).ready(function() {
    var days = [];

    $('#calendar').datepicker({
        dateFormat: 'yymmdd',
        showWeek: true, showOtherMonths: false, selectOtherMonths: false,
        navigationAsDateFormat: true, prevText: 'MM', nextText: 'MM',
        onSelect: function(d) {
            var i = $.inArray(d, days);

            if (i == -1)
                days.push(d);
            else
                days.splice(i, 1);
        },
        beforeShowDay: function(d) {
            return ([true, $.inArray($.datepicker.formatDate('yymmdd', d), days) == -1 ? 'ui-state-free' : 'ui-state-busy']);
        }
    });
});
</script>

NOTE: You can prefill days with a list of dates like '20190101' with a piece of code in PHP.

Add 2 lines to your CSS:

#calendar .ui-state-busy a {background:#e6e6e6 !important;}
#calendar .ui-state-free a {background:none !important;}

To get the list of days selected by the calendar in a <form>:

<div id="calendar"></div>
<form method="post">
<input type="submit" name="calendar_get" id="calendar_get" value="Validate" />
</form>

Add this to the <script>:

    $('#calendar_get').click(function() {
        $(this).append('<input type="hidden" name="calendar_days" value="' + days.join(',') + '" />');
    });

Apply implode on the string in $_POST['calendar_days'] and map strtotime to all the formatted dates.