How to launch the beforeShowDay of DatePicker from another function

I have a Datepicker with the following code. I have a dropdownbox and upon a change in the dropDown box I want to launch the datePicker beforeShowDay function. How can I do this?

var freeDate;

Part of my code is as follows:

$("#myDiv").datepicker({
    showOtherMonths: true,
    selectOtherMonths: true,
    dateFormat: "yy/mm/dd",
    onSelect: function(dateText, inst) {},
    onChangeMonthYear: function(year, month, inst) {
        $.ajax({
            contentType: "application/json; charset=utf-8",
            dataType: "json",
            url: '@Url.Action("ListDates", "Party")',
            data: {
                date: new Date(year, month - 1, 1).toString("yyyy/MM/dd"),
                userID: userID
            },
            success: function(data) {
                freeDate = eval(data);
            },
            error: function(request, status, error) {}
        });
    },
    beforeShowDay: function(dateToShow) {
        var returnResult = new Array();
        returnResult.push(true);
        var itemMatched = false;
        $.each(freeDate, function(key, value) {
            if (new Date(parseInt(value.substr(6))).compareTo(dateToShow) == 0) {
                itemMatched = true;
                returnResult.push('timeNotFree');
                return;
            }
        });
        if (!itemMatched) returnResult.push('');
        returnResult.push('');
        return returnResult;
    }
});​

$('#myList').change(function() {
    userID = $("#userList > option:selected").attr("value");
    // myList is used to have freeDate and the DatePicker must be shown accordingly.
    $.ajax({
        contentType: "application/json; charset=utf-8",
        dataType: "json",
        url: '@Url.Action("ListDates", "Party")',
        data: {
            date: new Date($("#myDiv").datepicker("getDate").toString("yyyy/MM/dd")),
            userID: userID
        },
        succes