这是我的解决方案.阅读评论.

(function($){


/** 
 * Returns a dictionary,where the keys are the day of the month,* and the value is the text.
 * @param year - The year of the events.
 * @param month - The month of the events.
 * @param calendarID - Events for a specific calendar.
 */
function getMonthEvents(year,month,calendarId){
  return {11: "My birthday.",23: "My anniversary" };
}

// Receives January->1
function addTipsys(year,calendarId){
   var theEvents = getMonthEvents(year,calendarId);
   var theDateLinks = $('#' + calendarId + ' .ui-datepicker-calendar a');
   for(eventDay in theEvents){
      // Minus one,because the date in the tipies are regular dates (1-31)
      // and the links are 0-based.
      theDateLinks.eq(eventDay-1)  // select the right link
         .attr('original-title',theEvents[eventDay])  // set the text
         .tipsy();   // init the tipsy,set your properties.
   }
}

// Because the the event `onChangeMonthYear` get's called before updating 
// the items,we'll add our code after the elements get rebuilt. We will hook 
// to the `_updateDatepicker` method in the `Datepicker`.
// Saves the original function.
var _updateDatepicker_o = $.datepicker._updateDatepicker;
// Replaces the function.
$.datepicker._updateDatepicker = function(inst){ 
   // First we call the original function from the appropiate context.
   _updateDatepicker_o.apply(this,[inst]); 
   // No we can update the Tipsys.
   addTipsys(inst.drawYear,inst.drawMonth+1,inst.id);
};

// Finally the calendar initializer.
$(function(){
   // Creates the date picker,with your options.
   $("#datepicker").datepicker();
   // Gets the date and initializes the first round of tipsies.
   var currentDate = $('#datepicker').datepicker('getDate');
   // month+1 because the event considers January->1
   // Last element is null,because,it doesn't actualy get used in the hanlder.
   addTipsys(currentDate.getYear(),currentDate.getMonth()+1,'datepicker');
});

})(jQuery);

不便之处：

>当用户从可见月份中选择一天,或者通过datepicker(‘setDate’,theDate)设置日期时,也会调用_updateDatepicker方法,这可能效率不高.
>它依赖于Datepicker的私有函数,如果在将来的版本中他们决定更改它的功能,或者更改名称,则此代码将中断.虽然由于功能的性质,我不认为很快就会发生这种情况.

注意：
我的第一种方法是挂钩ui.datepicker的onChangeMonthYear事件,但由于事件被触发,在替换日历中的日期之前,addTipsys方法会将tipsy添加到即将获得的日历日期清除.因此需要在元素刷新后调用addTipsys事件.

很容易：将方法挂钩到日历的onChangeMonthYear事件,然后执行setTimeout来调用tipsy.需要进行一些验证.