I have run in to issue that days that have to be disabled are shifted to the next day. The idea is that day that does not exist in our booking object or have a value less than 1 should be disabled on calendar.

here is simplified version of my script and demonstration on jsfiddle:

var bookings = {
    "2012-09-01": 24,
    "2012-09-03": 31,
    "2012-09-05": 27,
    "2012-09-06": 9, 
    "2012-09-07": 18, 
    "2012-09-08": 0, 
    "2012-09-10": 20, 
    "2012-09-12": 19, 
    "2012-09-13": 0, 
    "2012-09-14": 9, 
    "2012-09-15": 24, 
    "2012-09-17": 19, 
    "2012-09-19": 28, 
    "2012-09-20": 15, 
    "2012-09-21": 12, 
    "2012-09-22": 25, 
    "2012-09-24": 19, 
    "2012-09-26": 0, 
    "2012-09-27": 0, 
    "2012-09-28": 0, 
    "2012-09-29": 0
};

function MyEvent(date)
{
    bookings = bookings || {};
    this.date = date.toISOString().substr(0, 10);
    this.display = (typeof bookings[this.date] == 'number' && bookings[this.date] > 0);
    return this;
}

MyEvent.prototype.toArray = function () { 
    return [this.display, null, null]; 
};

$(function ()
{
    $('#eventCalendar').datepicker({
            dateFormat:    "yy-mm-dd",
            firstDay:      1,
            defaultDate:   "2012-09-24",
            beforeShowDay: function (date)
            {
                return new MyEvent(date).toArray();
            }
        }
    );
});

Can some one suggest me what am I doing wrong or is it a bug?