In chapter 4, of "jQuery UI Cookbook", effects are applied to days in addition to months. This isn't the intended behavior. The following code should properly differentiate between month adjustments, and other adjustments. Thanks to Dave Sharman for pointing out the issue.

jquery.ab.datepicker.js

(function( $, undefined ) {

$.extend( $.datepicker, {

    monthAdjusted: false,

    _adjustDate: function ( id, offset, period ) {

        var _super = $.datepicker.constructor.prototype;

        // Tell _updateDatepicker() that the month is being adjusted.
        if ( period === "M" ) {
            this.monthAdjusted = true;
        }

        _super._adjustDate.call( this, id, offset, period );

    },

    _updateDatepicker: function( inst ) {

        var self = this,
            _super = $.datepicker.constructor.prototype;
        
        // Don't do anything if we're not dealing a month change.
        if ( !this.monthAdjusted ) {
            return _super._updateDatepicker.call( this, inst );
        }

        this.monthAdjusted = false;

        inst.dpDiv.fadeOut( 500, function() {
            inst.dpDiv.fadeIn( 300 );
            _super._updateDatepicker.call( self, inst );
        });

    }

});

})( jQuery );