Here's how to highlight specific dates with custom styles in jQuery UI Datepicker:
Basic Solution
$(function() {
  $("#datepicker").datepicker({
    beforeShowDay: function(date) {
      // Define dates to highlight (format: [Y,M,D])
      var highlightDates = [
        [2023, 11, 25], // Christmas
        [2023, 0, 1],   // New Year's Day
        [2023, 6, 4]    // Independence Day (example)
      ];
      for (var i = 0; i < highlightDates.length; i++) {
        if (date.getFullYear() === highlightDates[i][0] && 
            date.getMonth() === highlightDates[i][1] && 
            date.getDate() === highlightDates[i][2]) {
          return [true, 'highlight-date', 'Special Day'];
        }
      }
      return [true, ''];
    }
  });
});
CSS Styling
.highlight-date a {
  background-color: #ff5252 !important;
  color: white !important;
  border-radius: 50%;
  font-weight: bold;
}
.event-date a {
  background-color: #2196F3 !important;
}