Highlight dates in jquery UI datepicker

Dates in JS are instances of object Date, so you can't check properly if dates are equal using == or ===.

Simple solution:

var your_dates = [
   new Date(2017, 4, 25),
   new Date(2017, 4, 23)
];

$( "#some_selector" ).datepicker({
    beforeShowDay: function(date) {
        are_dates_equal = d => d.getTime() === date.getTime();
        if(your_dates.some(are_dates_equal)) {
            return [true, 'ui-state-active', ''];
        }
        return [true, '', ''];
    }
})

Have a look at the documentation.

beforeShowDay The function takes a date as a parameter and must return an array with [0] equal to true/false indicating whether or not this date is selectable, [1] equal to a CSS class name(s) or '' for the default presentation, and [2] an optional popup tooltip for this date. It is called for each day in the datepicker before it is displayed.

This means that you need to create a function that will take a date and return an array of parameters where values are:

1. boolean - indicates if date can be selected
2. string - name of the css class that will be aplied to the date
3. string - an optional popup tooltip for this date

here is an example:

var your_dates = [new Date(2011, 7, 7),new Date(2011, 7, 8)]; // just some dates.

$('#whatever').datepicker({
   beforeShowDay: function(date) {
      // check if date is in your array of dates
      if($.inArray(date, your_dates)) {
         // if it is return the following.
         return [true, 'css-class-to-highlight', 'tooltip text'];
      } else {
         // default
         return [true, '', ''];
      }
   }
});

and now you can add the style to highlight the date

<style>
   .css-class-to-highlight{
       background-color: #ff0;
   }
</style>

I solved the issue using

var disabledDays = ["2011-7-21","2011-7-24","2011-7-27","2011-7-28"];
var date = new Date();
jQuery(document).ready(function() { 
    $( "#dateselector").datepicker({ 
        dateFormat: 'yy-mm-dd',
        beforeShowDay: function(date) {
            var m = date.getMonth(), d = date.getDate(), y = date.getFullYear();
            for (i = 0; i < disabledDays.length; i++) {
                if($.inArray(y + '-' + (m+1) + '-' + d,disabledDays) != -1) {
                    //return [false];
                    return [true, 'ui-state-active', ''];
                }
            }
            return [true];

        }
    });
});

Found the most voted answer not working. Updated the code little bit to make it working. $.inArray() mostly search for indexOf(). Also we can't compare two dates directly for equality. Reference : Compare two dates with JavaScript

function inArrayDates(needle, haystack){
    var found = 0;
    for (var i=0, len=haystack.length;i<len;i++) {
        if (haystack[i].getTime() == needle.getTime()) return i;
            found++;
        }
    return -1;
}

And update the accepted function as

if(inArrayDates(date, markDates) != -1) {
            return [true, 'selected-highlight', 'tooltip here'];
          } else {
             return [true, '', ''];
          }