How to disable specific dates in yui3 calender

I know how to disable past dates in yui3 calender but how can I disable specific dates ?

Should I specify rules for that ? their documentation is not very clear I can't understand the syntax they written for defining rules. If defining a rule is solution then how can I define one to disable specific date ?

ex : 25-12-2015

Indeed rules are a good way to disable specific dates, you can achive that like that:

var calendar = new Y.Calendar({
    date: new Date("2015-12-25"),
    customRenderer: {
        rules: {
            2015: {
                11: {
                    25: "mydisableddays",
                    10: "mydisableddays"
                }
            }
        }
    },
    disabledDatesRule: "mydisableddays"
}).render();

Since you most likely will have an array of dates you want to disable, you may use this helper function to convert your array to rules.

function datearray2filter(dates) {
    var ret = {};
    for (var i in dates) {
        var d = new Date(dates[i]),
            y = d.getFullYear(),
            m = d.getMonth();
        if (!ret[y]) ret[y] = {};
        if (!ret[y][m]) ret[y][m] = {};
        ret[y][m][d.getDate()] = "mydisableddays";
    }
    return ret;
};
console.log(datearray2filter(["2012-12-25", "2012-12-10"]);

Here's a fiddle for that https://jsfiddle.net/fxaeberhard/5e1b8ay9/6/