Месяц: Апрель 2018

<input type="text" value="2018-04-28" data-date-format="yyyy-mm-dd" />

.selected {
  background: orange !important;
  color: white !important;
}

var items = [
  new Date('2018/04/15').toISOString(),
  new Date('2018/04/16').toISOString(),
  new Date('2018/04/17').toISOString(),
  new Date('2018/04/18').toISOString()
];

$('input').datepicker({
  beforeShowDay: function(date) {
    var r = {
      enabled: true,
      classes: 'selected'
    };

    return items.indexOf(date.toISOString()) != -1 ? r : {enabled: true};
  }
})

тест https://jsfiddle.net/polyerter/j4npryje/110/