drkrol,
<!DOCTYPE html>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<style type="text/css">
.flatpickr-day.busy {
background-color: #FF0000 ;
}
</style>
</head>
<body>
<div id="slider"></div>
<link rel="stylesheet" href="https://unpkg.com/flatpickr/dist/flatpickr.min.css">
<script src="https://unpkg.com/flatpickr"></script>
<input class="flatpickr" type="text" placeholder="Select Date..">
<script>
var data = ["2016-12-12", "2016-12-16", ["2016-12-28", "2016-12-30"]];//
flatpickr(".flatpickr", {
onDayCreate: function(dObj, dStr, fp, dayElem) {
var a = dayElem.dateObj.getTime();
a = data.some(function(b) {
return typeof b == "string" ? a == fp.parseDate(b).getTime() :
a >= fp.parseDate(b[0]).getTime() && a <= fp.parseDate(b[1]).getTime()
});
if (a) dayElem.classList.add("busy")
}
});
</script>
</body>
</html>